IMHO accusations should be (but apparently aren't) against a punishable offense.
Accusations is a punishable offense
I like Rakanishu's approach.
I never play well enough to be accused of cheating, but if it happened I would feel flattered.
I would send a message back:
"Dear opponent, I want to send you a personal thank you note. Being accused of cheating is a great compliment on my play and believe me, this does not happen often. I am incredibly flattered that you were so impressed with my chess games that you think it reaches a level of ELO 3000."
