Due to the plugin OnlyProxyJoin failing in recent updates I decided to put together a quick replacement for the plugin. What it does it kick the player on connect if their IP does not match any entries in a list which is editable from the config.yml file.
Let's say you want to prevent players from connecting directly to your servers and force them to use your BungeeCord proxy. Well, with this you can. Add your BungeeCord's IP to the IP list in the config file along with the IPs of every other server set up with that BungeeCord. Then people will be forced to connect using one of the defined IPs.