As the title says - is there any way to avoid doing this?

Same problem, I'm 3 turns away from purging the continent of an AI player and I can't end the turn because I have to force their surrender...I don't want to force them to surrender I want to purge them!!!

Some workaround...

* Be an obnoxious bully before the war, attack their scouts, religion (even if it is shit), build up to lines, trespass with a lot of single units (dumb but it works) to build up grivieouses against you..

* Try to harras the cities and raze them, a lot of cities are just too expensive to win over.

* More then just a single war if there are a lot of cities, remember you don't have to take them over. Raze them for lots of gold, build a camp on it and keep raving.

They Attempt to make it more "realistic" with that occupation refuses to surrender even after the war, poorly implanted since it makes no sense when they culture/religion are 100% converted and so on. I think they are inspired by Euniver other games with "casus belli" so it is not just a flip over surprise war out of nowhere, but it is not as great implanted.

To give an example of how "bad" it is.

The worst part is that if they lose another war into a forced surrender and you are attacking them and occupied on another front, if they become vessels. You are obligated to give everything back and forced into peace. I lost 5 occupied cities because of this, lost a lot of war support to start another war.

Also you can lose war support if you load a save, I think it is a bug.