Throat Punch( Complete Scoundrel, p. 81)
By making a precise punch to the throat, you can render a target unable to speak effectively.
Improved Unarmed Strike (PH) , sneak attack +3d6,
Your successful sneak attack delivered with an unarmed strike temporarily hinders the target's ability to speak. For the next 3 rounds, the target takes a -5 penalty on any skill check requiring speech and has a 50% chance of failure when casting a spell with a verbal component or activating a magic item with a command word. Multiple uses of this feat don't increase the duration beyond 3 rounds. Using this feat reduces your sneak attack damage by 2d6.
You must be able to reach your foe's neck to use this feat.