If they ARE breaking the rules, we don't really care about that. They need a punishment that will deter those actions. If all that happens is they get pushed out of a FW (After taking advantage of the FW for however long they were in it before the action was taken) there is no discouragement. Run into FW for 12-24 hours and get pushed out again after you've had a chance to get troops home and everything.
If kingdoms are concerned with dying off due to this change, don't break the rules.