Say no without a list of reasons. Every reason is something they can argue with.

Reasons feel polite, and they are also a set of handles. Give three and the other person solves one, then another, and now you are negotiating over whether your refusal was valid in the first place. The decision was yours to make. Explaining it turns it into a claim that needs supporting.

Try one plain no this week with nothing attached. I cannot do that, or that does not work for me, said kindly and then left alone. If you are asked why, repeat the same words rather than reaching for a better excuse.