I seem to remember reading somewhere that a cult or group exists that seeks out Ioun Stones to destroy them. As a GM I'm using this as part of the reason I don't allow characters to have Ioun Stones during character creation. Can someone help me out with the name of this cult or did I just dream it up?
I'm not familiar with any such cult, and I don't see anything mentioned on the Pathfinder Wiki ioun stone article.
Of course, you certainly can create such a cult as a GM, and you could potentially write some interesting story lines around it.
If your goal is to restrict ioun stones, you can either use the wealth by level guidelines (ioun stones are expensive--and a cult trying to collect and destroy them would drive up the price) or just say "no ioun stones".