One way of getting to "no" is to put forward a high price, particularly if it's something you don't actually want to do.

What I like to do is put a number at about double what I consider it to be worth. This basically reflects the fact that I'd be spending weekends working, but also isn't so high as to be insulting - we're not talking huge numbers here.

The upside is they might say "yes", in which case you're getting what you consider a good rate for your time.

The downside is that they might say "yes", in which case they may expect higher quality for the price that you put forward.

This is also troublesome if you don't know the going rate, and unintentionally put a low-ball offer in.

Clearly if you want to just say "no", and have no interest whatsoever, then telling the client "no... but" is probably a better option.