I think Mastodon is resilient (because of the federated network) and responsive — just send a message (a post that's read only by the addressee) to the admin of your local instance and that should end the troll for good.

Even with the sign-ups (my instance went from 3K to 30K in a week), I have found my own admin very responsive.

Of course, the Mastodon approach to some extent offloads detection and reporting to individual users — but that also means more eyes on the problem. And, truthfully, the admin will I'm sure be quickly responsive. Try it, and let me know what happens. (Of course, if you don't notify the admin, probably nothing will happen until and unless the bad actor trolls another person on your instance.)