[Psycho]

Using ghost riders or some such to spend gems when storming is a annoying, but making a house rule that forbids it is not good IMO, as there is too much gray area. Lets say I sent an AQ with the aegis and armor of virtue to attack your force before storming. Was the intention to do as much damage to you or was the intention to make you spend gems? How can you know? Say I knew you had mostly demon MR 15 troops and you had a mage in the army capable of casting antimagic, which would make the aegis totally ineffective. Maybe I am just a poor player and didn't realize I can't hurt you and you spending all gems was an unintended side-effect. Or maybe I am a good player and was counting on luck, body ethereal and mirror image on AQ to make her stay longer on the battlefield until your mages spend all their gems.

There are several solutions to the problem of gem spending:
1) Don't storm the castle if it's not that crucial to take it.
2) Put twice the amount of gems (or three times if you expect both a magical attack and a regular one before storming) and cast spells that will put your mages to 200 fatigue, so by the time they recover ghost riders are defeated. All the major buffs and BE fall into this category.
3) If you have time, don't storm at once. Let him waste several ghost riders. Make him think you won't storm at all and then storm.

Yes, it makes storming forts in lategame terribly hard. But, I like it that way. It should be easier to defend than to attack. And if your enemy is storming such a well guarded fort, then it is probably your capital and he has already won.