Being polite to pushy people doesn’t mean doing what they push.

Politeness and compliance get bundled together, so it feels like a warm tone has to come with a yes. It does not. You can be perfectly pleasant and still not do the thing, and the pleasantness is often what makes the no easy to accept. Pushy people rely on your discomfort doing the arguing for them.

Practise the pairing out loud: thanks for asking, I am not going to. Same tone as talking about the weather, no change in volume. Repeat the second half unchanged if they keep going, since repetition works better than new reasons.