Yes, stopper is a Scrabble word!

Verb

1. Close or secure with or as if with a stopper

Noun

1. An act so striking or impressive that the show must be delayed until the audience quiets down

2. A remark to which there is no polite conversational reply

3. (bridge) a playing card with a value sufficiently high to insure taking a trick in a particular suit

4. Blockage consisting of an object designed to fill a hole tightly