It's pretty easy for someone to get unbanned if really they want to use the site as intended. Failing that, the vouching system gives banned users' good comments a fair shot at being restored, so even the worst case isn't that bad.
If you then click the timestamp of a dead comment you'll see a [vouch] link. If enough people click that it'll change the post from dead to live. Mods get to see vouches, and people who get enough vouches I think get converted from banned to unbanned.
I think there's a karma threshold four vouching, but I don't know what that is.