You can say your boundary kindly and clearly in the same breath. Soft voice, firm words.

Warmth and firmness get treated as a trade-off, as though a limit has to be either apologetic or hard. They are actually independent: the tone can stay gentle while the content does not move at all. Most of the harshness people fear comes from having waited too long to say anything.

Write one sentence for the limit you need to state, then check two things: is it kind, and is it unambiguous. Adjust whichever is missing without weakening the other. Say it early, before you are annoyed, because that is when both come easily.