gfm
Refuses to Talk!!!
But the problem is that conquering them entirely is often difficult because they'll cozy up with usually another powerful civ (who'll force you to declare peace or fight a bigger war). I suppose you could say "Screw it" and try to take out the one civ while holding off the other and then sue for peace, but that's often a roll of the dice I'm not willing to take. Then again, I freely admit to being weak on warfare (and the associated production/economy management during wartime).
I've been there before, and war is not my forte (even though I'm not the peaceful builder I once was), but I find the challenge entertaining. I also like the fact that a losing nation will try to suck up and offer itself to a larger power to avoid being completely destroyed.
To tell the truth, I often watch the diplomatic situation pretty carefully to avoid being trapped in such situations, even bribing other nations to go to war with someone else before I declare war myself. They're less likely to accept a vassal with an asterisk attached (that asterisk being war with ME !) if they're already at war with someone else.