A fuzzy issue of responsible disclosure

If you want the fuzzer users to stop filing reports, that's up to you. You can just tell them to stop filing reports, or block them from your mailing list if it comes to that, no need for the police to get involved. But at least some of them will still issue reports to the general public, likely with no embargo (as you have declined to accept their reports in private), so that end users can take reasonable security precautions. What effect this has on your software's adoption is, frankly, your problem. You can't label such disclosures as "harassment."