“Try Finite Incantatem, that should stop the rain if it’s a hex or curse; if it doesn’t, something’s gone wrong with an Atmospheric Charm, which will be more difficult to fix, so as an interim measure try Impervius to protect his belongings—”
-- Hermione Granger suggesting ways that Ron could stop the rain in Yaxley's office (DH12)
The General Counter-Spell, Finite Incantatem, is an all-purpose spell which stops currently operating spell effects.
References from the canon
- Snape used this spell to end the various unfortunate spells that were affecting members of the Duelling Club when things got a little out of hand (CS11).
- Lockhart thought the incantation was "Stop! Stop!" (CS11)
- Undercover at the Ministry of Magic, Ron asked Hermione what spell to use to stop it from raining inside Yaxley's office and she suggested Finite Incantatem (DH12).
- A question about the spell was used on the first W.O.M.B.A.T. test, and the answer is probably "false": "'Finite Incantatem' should be used as a precaution when a Muggle rings the doorbell."
Commentary
Etymology
"finio" L. settle, end, die + "incantationem" L. the art of enchanting
Notes
Contrary to what is shown in CS/f, this spell does not blow up rogue Bludgers. The magic required to tamper with a Bludger is extremely high level and can't be counteracted a simple spell, especially not one cast by a 13-year-old (CS/f).
This spell is referred to only by its incantation in most of canon, but on Pottermore it appears in the Standard Book of Spells Grade 2 under the title General Counter-Spell (Pm).