First of all: I am not a restaurant magician so anything I say is based on "I imagine it should be a little like this".
As the others point out, you are there for their entertainment. This means they will remember you by how good you make them feel.
Your aim is, to do that with magic. Your tools are the effect and the patter.
Good times include laughter so you can put humor in your patter.
Now, as you notice, the moment you introduce yourself as a magician, they'll link it to disappearing stuff, hence their joke about the check.
If you introduce yourself as an entertainer, they will wait for the way you're gonna do that. So you'll have finished a routine before they get a chance to joke about the check and probably won't at all.
That is your first out: preventing them from making the link.
If they do make the joke: Let it work. Let them have their laugh (this will add to their perception of a funny experience and you don't even have to do anything yet!
). When the laughter starts to die down again, you can rekindle it with your witty comment, doubling their laughing experience!
So far my theory. Now for the comment, I was thinking:
Guest: "Can you make the check disappear??"
You: "Haha, well sir, I stopped doing that because after a couple of checks, the owner/my employer threatened to make my contract disappear
You: "But I can show you the following...."