Seems like gifts may be the way to go then. I would try killing the person, but the whole situation is strange. In the war that is currently happening, the character who has the claim is not even fighting. A lower baron declared war on a duke over the title, but the title claimant has not joined the war because he has his own issues. I actually didn't want to kill either of them because they're family members. He also doesn't have a proper heir yet. The worst part is that since this happened, I tried giving the defender more power because I had always planned on him becoming Despot of Africa as a distant member of my family, but it didn't seem to help him. He simply doesn't raise any extra troops.
Edit: I gave the bastard 200 gold in gifts and he is not using it to win the war...
Edit 2: I gave him thousands of gold just to see what would happen and he still wouldn't use it. I get a feeling this is a bug. He actually went on to lose the war while fast forwarding with thousands in the bank. This is silly.