Questions that could be asked:
’Am I legally obliged to remove this sticker?’
’Am I legally obliged to tell you who put up this sticker, if I have that information?’
’Am I legally obliged to explain my beliefs to you?’
‘Am I legally obliged to continue this conversation?’
If the answer to any of the above questions is ‘Yes’, move to, ‘What is the legal basis of that obligation?’
Then say you’ll get your own solicitor to verify, and please can police submit that request in a letter, because you’re not.