I voted "I like this plugin idea" simply because it's about the freedom of server owners to do what they want.
When someone pays for a dedibox or similar, it's up to them to decide what goes on in that box.
Do they want F2P people there, F2P people gets to go there.
If they don't want F2P people there, then the F2P people doesn't get to go there.
It's not your "right" to play on anyone's server. I mean, when we're on the subject, how about banning password-protected servers? I mean, it's like discriminatory! Just because I don't happen to belong to the group of people knowing the arbitrary string of letters I can't join!
VAVLE BAN PLZZZZZZZ!
You know, the sense of entitlement some people have... really...
No wonder you can't ban people anymore without having to deal with a DDoS these days :/
I personally don't mind F2P:s, far from it (they're great fun to backstab repeatedly), and some of my good friends play for free. I mean, as far as I'm concerned this plugin might as well not exist, because people can, as others have mentioned, join more welcoming servers.
So, please drop that attitude. You have absolutely no rights to play on anyone's server. They spend time and money (servers are expensive) in order to keep their server up, because they are nice. Every time you're playing on a server, always know that there's a LOT of work behind it.
Though, if we're talking plugins that shouldn't exist, how about that playercount-spoofing thing? Or the thing that assigns various names and pings to bots, so you think you're playing on a server with people on it.
That if anything, is unethical.