“That won’t work for me” is a complete answer. You don’t need to bring a lawyer’s argument.
LEADERSHIP
LEADERSHIP
Reasons feel like what makes a refusal legitimate, so you supply them. The problem is that a reason invites a rebuttal. You have turned your no into an opening statement and handed the other person a way to win. A plain decline has no edges to grab, and it is usually true.
Practise the short version on something small this week, an invitation you do not want or a favour you cannot fit in. One sentence, warm tone, full stop. If they ask why, you can repeat the same sentence rather than upgrading it.
Which refusal did you over-explain when a sentence would do?
Free, with nothing to buy inside it.