Here's one that I've been willing to try out: Although I hear a lot of people say that, I don't think I have met many people who were truly not cut out for mathematics. The problem is more that most people haven't gotten the chance -- or given themselves the chance -- to get to know mathematics. If you can think logically, you can do mathematics.
I really believe this is true. So instead of taking their remark as an insult, try to do something constructive with it. Maybe apply the Socratic Method to make them solve a small problem. This might take part of the cause of their disrespect/lack of interest away.