Just say no. That’s all. “No.” Don’t have to explain anything or make any excuses or name other charities you give to. Be firm.
If they push it tell them it’s bordering on workplace harassment and that you are taking notes on the issue and may have to seek remedy. Be careful how you say that. Do not mention the word “legal”, “lawyer”, “court”, “suit” or such.
Do take notes, dates, times, names and a summary of what was said and how it was said. If you are firm, brief, polite and unemotionally firm on the issue it is unlikely to go further.
If they push beyond that level, then do see an employment lawyer and get the free initial consult as to what to do next.
Good advice...thanks for posting.