Direct opposites:
* Don't - This is the most direct opposite, but it's usually used in a negative command.
* Must - Implies a demand or obligation, the opposite of a polite request.
Less direct opposites:
* Demand - This implies a forceful request, the opposite of a polite one.
* Insist - Similar to demand, but implies a stronger assertion.
* Command - This conveys an order, completely opposite to a request.
Informal opposites:
* No way - This is a casual way to reject a request.
* Forget it - This implies a refusal to consider the request.
The best antonym for "please" will depend on the specific situation and the intended meaning.