But it's not a debate. They have no interest in listening to you,
Adam doesn't debate Bob to convince Bob of Adam's viewpoint. Adam debates Bob to convince their shared audience: Charlie, David, Edward, Frank, George, Harry...
When Bob is ejected from this forum, Charlie, (et al), never hear that debate and are never convinced of Adam's views. They aren't inoculated against Bob's logic. When they come across Bob uncontested in another forum, they may be persuaded; they fall into his echo chamber. When they bring their half-formed ideas back to your forum, they are banned as apologists rather than debated.
Ejecting Bob makes your forum better. Adam debating Bob makes the world better.
Fascism demands a response. When it stands without objection, it grows. We are obligated to respond, less to convince the fascist to change their ways, and more to extend a branch to those passers-by who might get swept away.
Sometimes that responder is me. Sometimes it should be you.
It's the only worthwhile response. If you try to debate them, they will just toy with you and give garbage arguments, then go silent when they run out of useless arguments.
The response in this context is a preemptive: "not welcome here". We've all heard what the fascists have to say, and it's worthless. Zero tolerance for fascists.
That may be true, but have you considered the space lizards? They're the ones controlling the whether or not your child gets an armed babysitter or transed when they use the litter box at school.