ProtocolManagement is a plugin which will allow you to either block or allow a specific or multiple protocols.
Tested with the latest Spigot as of 1st October 2014, any Spigot build which supports the 1.7/1.8 protocol should work but is not tested.
Code (Text):Whitelist: false
Message: "&6&lPlease use 1.7"
4: 1.7.1 - 1.7.5
5: 1.7.6 - 1.7.10
Other Protocols: http://wiki.vg/Protocol_version_numbers
GitHub Repo (Source): https://github.com/connorlinfoot/ProtocolManagement