Spawnrape really kills the fun in a battle imo. The only reason why you would spawnrape is to kill the population of a fief you just took and leave burned earth, so just taking the fief, killing everything and then move on while leaving nothing in the fief itself.
I think it would be nice to add a new attack mode on fiefs instead - pillage. Once you win you automaticly kill the population in a fief, maybe also destroy all unspent production points the fief has stacked so far?
If we get such an option it should be forbidden to spawnrape the last standing flag in a fief - it should be allowed if one flag gets spawnraped but other once can still spawn normal, since it then is a valid tactic to kill some defenders quickly.
On open field it shouldnt be allowed in general imo, once you drop all the flags the whole army is going to die anyway.