Yeey, my mom just called me that when I got home I'd have to fix the neighbour's TV. Because after 10 hours of school the only thing I want to do is fill in the role of a free tech support.

This is going to be a bit tricky, but repeat after me: "I'm sorry, but I won't be able to do so. I have other more pressing duties."
You could also shorten that with: "No."

In case you encounter the: "But I already said that you would!", Just respond with: "Then you shouldn't have made a promise that didn't concern you in the first place".

@Jilano I think saying the magical "no" is an art, which I very much would like to learn, I guess later on It will help me a lot.

@Besi Definitely. One must not abuse this power, but when used correctly, it can indeed do wonder.