A Revoke Invitation Ritual was a basic ritual intended to revoke the invitation to vampires. First seen in "Passion", it was used to revoke Angelus' invitation from Cordelia Chase' car, Willow Rosenberg's room, Buffy Summers' home and Rupert Giles' apartment. Buffy later had Willow and Tara Maclay revoke the invitations of Dracula, Harmony Kendall and Spike into her home. The vampire can be reinvited with no complications except the ritual would have to be performed again if the invitation had to be rerevoked.
Rupert Giles mentioned that the ritual required a recitation of a few rhyming couplets, burning of moss herbs, sprinkling of Holy Water, and hanging of crosses. Apparently the ritual doesn't need to be fully complete for the invitation to have been revoked as when Angelus was blocked from entering Buffy's house, Willow was just about to finish the ritual though presumably if she didn't finish the revocation would have failed.