You have already tried politeness. Politeness has failed. Now is the time for clarity. You must give the clearest answer you can. The best way to do that is to use a magic word:
No.
There are three rules for using this magic word in this context:
- You must say "No."
- "No" must be the first word of the sentence.
- "No" said by itself, is a complete sentence.
So, the best answer to the repeated question is:
No.
If this is too impolite for you, you may offer whatever polite filler phrases you wish, after you have said "No", like this:
No, thank you.
No, I don't want to do that.
No, I don't mix business and politics.
No, but good luck.
