The Grinch hates Christmas and wants to stop it from coming, so he forms a devious plan to impersonate Santy Clause" and to steal the Whos' Christmas presents. Come Christmas morning, the Grinch is in for a shocking surprise.
He did not stop Christmas from coming. The Whos were still out in the streets, singing and being merry. The Grinch realizes something new, that maybe Christmas doesn't come from a store, that maybe, Christmas means a little bit more.