You don’t. If people didn’t ever get banned here, this place would be overrun by nazis in no time.
If you do feel the need to call someone a nazi, then maybe just don’t, though. Report them, if they’re violating the rules. If they don’t, then either ignore them or feel free to engage in a discussion on the part you take issue with, without assuming they’re a nazi. Vilifying people is actively bad for convincing them to change their ways, and it’s a part of the Reddit culture which unfortunately made its way over here, too.