We have a machine that rezzes one object per agent in the area. It tries to be aware of the remaining parcel capacity and avoid rezzing when the parcel is nearly full, but that seems to be a tricky thing for our scripter to achieve. What would help is to disallow llRezObject from returning objects to inventory if the parcel gets full. It'd be better if the call just failed to rez anything. If a script absolutely must force rezzing then llRezObjectWithParams might add a FORCE flag.