The case of libel is easy to prove in terms of falsity, but the problem is one of damages. Proving your reputation was harmed is difficult in terms of monetary damages. It also requires hiring an attorney to sue, which is expensive. Sometimes a simple threatening letter from an attorney is all it takes to scare someone into compliance.
As a practical matter, and speaking from the perspective of both a lawyer, a youth coach, and athletic director for a grade school's athletic program, my advice to you would be to have your league officials . . . specifically those who gave you an interpretation of a rule upon which you acted . . . confront the coach who is disparaging you if he is under their supervision, or to reach out to the league officials who have authority over him, and have them clarify the issue for him to encourage him to stop acting like a bad sport. These league officials should be the first people he or you should go to regarding any dispute about game play in their league
In the alternative, as suggested by the prior lawyer's answer, you could bring an action in law (for money damages) and/or equity (for an order of court requiring him to stop calling you a cheater). Both claims would require you to prove that the other coach has published (written or spoken) statements that are false, and cause harm to your reputation. Under Pennsylvania law, you only have one year in which to bring such a claim. Failure to bring this claim within one year of the libelous/slanderous statements will result in your claim being untimely and it will be barred by the Statute of Limitations.
This answer contains general information only; and it is not intended as legal advice. It is not intended to and does not create an attorney client relationship. Information contained here is only a starting point and you should consider discussing your specific problem in depth with a licensed attorney.