You could try the 'force deplomacy' mod.

Also, when I face this kind of situation in campaigns that I'm not too serious about, I'll cheat. I usually hire an all merc army then march it into enemy territory using 'character_reset' and 'auto_win attacker' to kill ever single enemy army in sight. Once you destroy 90% of their army, they will give up and leave you alone for a decade or so. I always end my war with them (at least ignore them) until they rebuild so that I don't consider it too much cheating. But I only use this in campaigns that I would quit otherwise or I wasn't too serious about to begin with.