If the warning really was unnecessary, it's aggravating but, in the long run, harmless.
Saying "I didn't do it, and I certainly agree that I don't plan on doing it in the future" is usually the best you can do.
Some managers will be willing to include a statement from you to that effect in your file. If you go that route, keep it very calm and factual -- no accusations, no emotion, just "I don't believe this is a correct description of what happened or didn't happen." However, it is EXTREMELY difficult to write a self-defense note that doesn't come across as ... uhm... defensive, so it might be better to let it lie.
(I got in trouble in my first few years with the company over a quote that someone misunderstood. Talked it over with my manager, he talked it over with his manager, they agreed that the whole incident was "mostly harmless" and that if there was a lesson to be learned I had learned it, and we went on from there. I don't know what's in my file about it, but I don't see a need to worry about it either.)