I am going to assume you already have it working with tokens, subsequently removing functionality is rarely an issue.
Pull the power to the bill validator. Although depending on what you have for a platform the machine may detect the loss of functionality.A screw driver to remove the bill validator and a cover plate should do the job. Even if your just leaving it connected by hanging in the machine.
You could also do what the pinball guys do and put a metal bar across the front of the machine.
You could fill in the entry with spray foam or PL400