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.