When I have a poor prince, sieges are the best way to get rid of him, even if it is a mere fort. Just put him in range of the missiles coming out of the fortifications and put the speed slider on maximum. Soon he is dead. Even the worst morale princes do not run, as they never come close to the enemy - sure, they start to waiver when they lose most of their men, but I have never had a unit of Bodyguards run while sitting still in front of the walls. Field battles have way too much risk that the intended suicider will survive by virtue of his own cowardice. And once they run, it is even harder to make sure they stay long enough to get killed the next time.