Abraxas
Explorer
From Piratecat
In fact, it's even a good idea from a strategic level, because invaders trying to deal with this problem are going to require a spell (in this case a limited wish or a miracle, since the PCs don't currently know Filter), and that's one less spell that is going to be aiding their attack.
Are the PC's in question the Defender's?
If so wouldn't Velendo or Tao be able to cast Filter?
It is a 2nd level cleric spell.
Unless, of course, you have new Cleric spells discovered in a manner similar to new Wiz/Sor spells.