Prevent 1.8 clients from joining?

[xoft]

Did I say it'd be easy? Just wrote a quick plugin that should work for you:
https://github.com/madmaxoft/SingleClientVersion

Install it as usual, enable it on your server; upon load, it will create a SingleClientVersion.ini file, edit its contents to change the allowed client version, reload plugins to apply the changes.
I know the plugin's a bit raw, it has no description, no help, no user-friendly interface, but it's the very guts that can now be improved by anyone else.