Fediverse

Unfortunately, the harasser would be able to do what you said, and while the victim and instance administrators can try to mitigate it, the nature of pseudoanonymous platforms is that there is only so much you can do since the harasser can easily change IP address and other identifying markers and continue the abuse, so it can quickly turn into "whack-a-mole" where you try to ban offending accounts but others pop up. This is also true on non-fediverse sites, unfortunately.

If you are getting continuously harassed or especially if you are concerned for your safety, I would recommend reaching out to support organizations and/or the police.

You can report them to both your instance and the instance that is hosting their accounts. On mastodon you can also mask them, which I guess doesn't inform them of anything. So I would the mask them anytime they messaged me. It has never happened to me so I don't know how effective it is.

I mean a lot of this would depend on the situation. You could basically try to get their IP and maybe put it under the hashtag #fediblock, maybe other admins will block it, but seems unlikely. It would be nice if we could have a regex based block function, but we don't currently. You could change your username and picture to make it harder to find you, but I don't know what other tools you might have.