You can limit yourself to Local, which will only show you communities from your own instance.

Also, when you see a clear case of someone acting in a shitty manner you can report them. This report doesn't just go to the community's mods. It also goes to all the admins of all instances involved (your and their accounts', and the community's instance) as well as to the top admins. But be warned that it's not anonymous (your username appears in the report).

You can also open a discussion with the admins and users of your instance. Your admins can block other instances, but that will apply to all users of your instance.

I'd suggest just to be patient though. Usually fascist instances tend to get banned and defederated.

Or you can find an instance that is already defederated from the problematic one.