Your instance can block them. Looks like it doesn't block very many, though:
https://mastodon.cloud/about/more
But Mastodon as a whole is just a software project that's freely redistributable. It's effectively impossible to stop an instance from being created which allows anything. Other instances and individuals can refuse to federate with them, and that's basically how that's handled.
Other instances can shut them out. The bigger, better known ones tend to cooperate on this, particularly the ones operated by Mastodon project.
But unfortunately as Erin points out, there are no controls on who downloads open source software -- there almost by definition can't be.
Founder Eugene Rochko has made a number of policy statements on what he will and won't blanket ban on the instances he controls. For example, he decided not to blanket-ban Gab, which is a right-wing site forked from Mastodon. I'm in no position to summarize his reasoning. Don't know much about who administers your instance. There's some info on his/her profile: https://mastodon.cloud/@TheAdmin
@pieist @kissane "Oh yes, Mr Jones down the street hands out antisemitic posters and demands that synagogues be removed from the neighborhood and yes he bought a book on building bombs, I saw him reading it on the porch, and yes he has a roomful of guns, but...I choose to, you know, just block him. I dont speak to him. I ignore him. He's not there!"
@JohnShirley2023 @kissane In that situation, the company that manufactures the paper Mr Jones uses wouldn't be in a position to cut off his supply, and it really wouldn't fall to them to police him.
In the same way, Eugene Rochko literally cannot stop others from using his software. He can only say "I won't carry anything that comes from your instance and I might even go so far as to block other instances that do." But he can't stop them using it.
