I can't tell which instance they're on, but if you report something to admins, it's reported to your local server admin. If they're on a different server, the absolute best your admin can do is contact the other user's admin to forward the report and block the instance. The more likely outcome is that they block the other user from your server.
I don't know how responsive the admin of LW is, but it seems to be a site that's prioritizing growth and "openness", but, as far as I'm concerned, the real value of this space is in smaller servers and smaller communities, with federation just giving access to occasional trips to "big city" sites. Smaller sites usually have more responsive admins and are quicker to cut off bad actors because they care more about community health than unlimited growth and access.