Short answer: yes, they can. Regardless of what some websites are saying.

Mastodon is “an alternative to Twitter”. Mashable just wrote that:

• “Mastodon.. in many ways it’s like Twitter, but it is also so not like Twitter… What makes it stand out? It has better privacy controls than Twitter. Also, neo-Nazis are explicitly banned.”
• and then, 2 lines below: “Mastodon is a type of free and open source software (FOSS) known as “GNU social.”

No, sorry. Software of that kind cannot “explicitly ban Nazis”. Or anybody else, for that matter. Free as in Freedom software means “software that everybody can use in whatever way they want” (of course in compliance with applicable laws). Including being Nazi. Here is nobody less than Richard Stallman saying it.

The truth is different, and well explained in this thread:

“Mastodon the software is not politically aligned. mastodon.social the flagship instance of the developer (me) that simply runs/demonstrates the latest code, is, in this case, leaning left. You can run your own.”

I myself usually explain the same concepts pointing out that one can build the perfect police state with “Free as in Freedom” software. So, don’t go saying, or expecting, that Free as in Freedom, open software cannot be used by Nazis.

Of course it can. And that’s a good thing. It’s what Freedom is all about. Paragraphing Stallman, see above, “We need to stop [x], but we can’t do it through software licenses”.

PS: if you’re interested in “alternatives to Twitter” please let me know what you think of the Percloud.