I think that placing the restriction of "no fame points gained by this action" would be pretty much ok.
Regarding forcing someone to kill you, when you attack someone with lethal force you /are/ basically saying: "defend yourself or die". It is not "execute me while I'm unarmed", but "use self-defense if you want to live". It is something every Hero is doing every time, though with very slight chances of death. What would be the problem? If the other player's unit is set to Defensive or Normal and you are at peace with him, you will have to be the aggressor (your fault), if it is set to Aggressive or Murderous then he /was/ picking up a potentially lethally (to the other) fight.
Don't you think?
