When I used to conduct defensive wars, I would accept a peace treaty as soon as I reached 100% warscore, but lately I've changed my strategy. Now, if the situation permits it, I ignore the aggressor's feeble attempt to stop the pain if I am crushing his military and either reject or ignore his peace treaty. Instead, I continue to siege his realm's holdings, earning a lot of additional gold through plunder. He will stay at 100% until you are ready to stop the war, there is no reason you need to stop it before you are ready. I have made a fair more gold since I've started doing this.