It’s technically possible but I can’t find a way to do it through the default web UI. The Lemmy API supports it, and using it you can point blank ban a user by user ID and community ID, even if they have not posted. You probably have to use a third party client to do so and I don’t know which one since I don’t use any of them (I am a desktop only nerd). You could probably also hand-craft the appropriate POST request if you really wanted to, but that sounds like work.

And Lemmy.ml appears to have a page with a web form on it that exposes the API hook for their instance, for example. I don’t know if Lemmy.world does, but I can’t find it so I suspect they don’t.