One of the core tenets of democracy is that the people get a choice in their government. If the current government is able to rip that choice away if it's unpalatable, then that's anti-democratic in and of itself.
If we all vote to ban the party some dude started to advocate for killing all left-handed people, we’re not being anti-democratic - we’re being serious people. If you think this party poses no threat, then that’s a real argument. “But it’s not fair that I can’t vote for the cannibals/Nazis/anti-democracy advocates” is unequivocally not (and it’s not more so just because you don’t think they’re actually Nazis or whatever).
Outlawing an action is fine. Outlawing a party who says the action should be okay is anti-democratic.