It's not clear to me how often someone from Plarium reads, acts on, or responds to these questions. Using the Menu always gives a response as far as I know.

If you haven't already, I suggest you go to the Menu and then select Ask A question - Question type - Violation of Terms of Use - Question issue - Complain about another player and then give them the information you've given above. There's no need to mention that you're going to quit if nothing is done about it.