Quite silly question (especially taking into account my hours played), but once I see notofication about enemy is close to Victory Condition X, what could I do to prevent his vistory? The only mean I found useful is to declare a war, destroy or block few star systems... but what if I'm peaceful faction?

Is there a way to effectively reduce or stop Dust income, technologies researching, exspansion etc with only economic or diplomatic solutions? Except spreading munity via hacking home systems.

AFIK no. But you can always try to win before AI by science.

Then it's really sad. Maybe victory conditions should be rethinked (e.g., "colonize X systems" -> "colonize X systems more than others") or new ways of influencing competitors should be done. I'll think about it.

Hey, here's a few things to try without using military: 

Dust Income 

Population Reduction

The easiest way to disrupt Dust income is to reduce population as nearly every Dust generator in the game relies on planetary population to generate it (you get +x Dust per pop on Y type of planet, etc). You can remove population by hacking to embed sleepers and abduct them. 

Government (Law) Disruption

You can hack to cause civil unrest with goverment changes (on capitals) or hack systems to support other types of goverments (changing faction sensitivity AWAY from Ecologists and Pacifists will help with changing their ruling political parties at next election and can remove some very powerful Dust generating laws (especially for Pacifists). 

You can also use Influence and Diplomatic Pressure to force them to do things like pay tribute or accept diplomatic conditions that are unfavorable for them. If they're using Minor Factions you can spread influence over those Minor Factions to peacefully absorb them and remove them from your opponent. Be advised that this will likely inspire military action against you. 

Disrupt Trade Routes

Any good economic victory is going to depend on trade routes. Establishing colonies between your opponents' far lying systems and then closing your borders is a great way to disrupt trade routes without using military. This forces their trade routes to go other ways or be blocked entirely. Be advised that this will most likely inspire military action against you. 

Science Production

The best way to stifle Science production from an opposing empire is to reduce population approval rating and build the science wonders before they do. (Namely the Research Park and ITER). 

Reducing Approval

This can be very tricky to pull off because approval is largely handled by the player themselves but there are a few things you can do to try and disrupt approval. Hacking provides opportunites for changing government types and influence political parties which can have some affect on population happiness. 

Reducing FIDSI Output

You can Influence to encroach on your opponents' systems and reduce their FIDSI output through diplomatic pressure. 

You can once again using hacking to disrupt goverment and law. To know the specifics of how to influence government tor law in order to accomplish this will require some familiarity with the law system and which political parties your opponent is likely using to catapault their science. The obvious one is you want to pull the scientific party out of favor by influencing systems away from scientists, the best political parties to influence support of to mitigate scientific growth are Religious and Industrials. 

Using Pacifism to your advantage

The most devious way to peacefully disrupt your opponent in every way is making sure that Pacifism is the leading political party in your government and force peace on others. While in peace you can continue to use hacking to influence political parties that get stifled by peace (such as Militarists and Industrialists) so they have very few laws they can use their advantage (for Dust and Science) or if they're going for Wonder then use Militarists or Religious. 

You can also use the Pacifist laws that reduce the cost of diplomatic decrees allowing you to more easily use influence to your advantage to force people into things that slow them down. 

Summary

In summary, your best options if you want to stay peaceful are hacking and influence. Use them both to disrupt government, cause unrest and stifle FIDSI production and trade routes. Pacifism will be your best political option if using Dust and Influence to accomplish this. Focus your FIDSI production on Dust and Influence, build colonies near your opponent so your influence spreads cause border tension and increase diplomatic pressure and use hacking to abduct population and affect your opponents' government options. 

Be advised that doing this will increase the likelihood of military action against you, so be prepared to defend yourself. 

So I could slow things down, but not roll them back. If an enemy is close to Conquest victory, I could try keep him busy with riots, and maybe he'll postpone further expansion, but I can't decrease number of his systems (until one of them is peacefully converted while at riots). Seems legit. Though connected to single game system, hacking.

Hacking and Influence (Diplomacy) are the primary weapons of a pacifistic approach to this game, yes. 

If you want to stop Conquest, the best option is to build colonies near your opponents' border, expand your Influence production to keep border tension diplomatic pressure up and then close your borders. This will force them to go to war with you in order to take your systems for the Conquest victory. This assumes you have enough systems to prevent Conquest victory without going through you. 

There's a way to do everything you're trying to do but it's much more difficult for a pacifist to succeed in a military galaxy. 

Valadeus wrote:

the best option is to build colonies near your opponents' border

New colonies just for sake of new colonies -> overcolonization -> decreased approval -> decreased incomes etc. Well, maybe. World is unfair, even if it's sci-fi world.

Thanks for your suggestions, Valadeus!

Overcolonization isn't going to hit you as much as you might think. There are a number of researches (three that I can think of immediately) that increase the number of colonies you can have before disapproval, you also get more as you unlock the Empire Development tree. You can also change your government type to Federation to get one more for free. You can mitigate the loss of approval with various laws, using Transvine as a system development luxury (for approval) and other things as well. If you're playing a pacifist approach to the game, you'll have plenty of time and resources at your disposal to mitigate the penalties for overcolonization. 

Everything in the system is intertwined, just have to look at each available system in the game to see if it helps you in any way with your goal. 

I hope this all helps!