A silence spell for either players or monsters or both. Not specified whether it would be area-of-effect or targeted at specific creatures. The main function of silence effects would be to prevent spellcasting. Not specified whether this would also affect reading scrolls (though probably not since you can “cogitate” the formula on scrolls if polymorphed into a speechless form).

However, this may require defining certain existing spells as not requiring speech, because it does not seem entirely fair to take away all of someone’s escape or healing spells with fairly low level magic.