Depends on the debate. The government can’t stop them from debating. If you host a debate though, without any government affiliation, you’re not forced to let them attend. In the same way, if you own a forum on the internet, you can silence anyone you want to. The literal ONLY thing the first amendment protects against, is government persecution for speech. It has nothing whatsoever to do with how people react to said speech. Just because someone cries “Muh first amendment!” doesn’t mean you actually have to respect them. You can shame them, silence them, ostracise them. Put up billboards telling people how fucking insane they are. Take out an ad in the local newspaper to display their bullshit in an attempt to shame them into submission, tell them to go fuck themselves. If they’re in your house, you can toss them out. YOU can do whatever you want. YOU can’t breach the first amendment, only the government can.