Muggles' Guide to Harry Potter/Magic/Langlock

Overview
The Langlock spell is one that prevents someone from speaking about something specific by binding their tongue to the roof of their mouth.

Extended Description
Harry finds this spell in the Potions text that belonged to the Half-Blood Prince. He uses it at one point in the story to silence Peeves, who is loudly commentating a fight between Dobby and Kreacher. We also guess that this is the spell left in the entrance to Number 12, Grimmauld Place by Mad-Eye Moody as part of the attempt to prevent Snape's entrance.