Force a C# method to pass only positive parameters?

Just make the parameter a uint or a ushort. That will prevent negative input at compile-time. It won't prevent 0, however... I assume you're really after non-negative input rather than strictly positive input.

If your method has to accept a signed parameter, then:

• You can validate the argument value, throwing an exception if it's invalid. (This is a much better idea than using Math.Abs to try to "fix" invalid input.)
• You could use Code Contracts to indicate to other code that the parameter should have a non-negative value, and it will attempt to prove that everywhere you call the method, the value will be non-negative. This won't be enforced by the C# compiler, but it can still be a build-time procedure.

If you go with the exception approach, then you should also have a comprehensive set of unit tests around the code calling your method - and that code should validate its inputs as well, of course.