Direct Opposites:
* Please: This is a request, which is the opposite of expressing gratitude.
* You're welcome: This is a polite response to "thanks," but it's not technically an opposite.
Other options depending on context:
* Disregard: If someone thanks you for something you don't want, you might use this.
* Ignore: Similar to disregard, this implies you don't appreciate the gesture.
* Decline: If someone offers you something and you don't want it, you can decline their offer.
The best word to use will depend on the situation and the specific meaning you want to convey.