I once played on a server that, if you had xray installed, it disconnected you and didn't allow you to connect till you cleared your .jar of these files. Is there any plugin out there or someone who can make a plugin like this?
If the plugin leaves a server digital signature (like zombe and CJB's mods) then they can be disabled from the server, and any players using them can be kicked, even if they have them installed. Below is an example of a plugin which does this: http://dev.bukkit.org/server-mods/cfbanner/ This is only possible due to the anti-cheat plugin creation capabilities of both these mods, they send a message to the server if they are enabled on the client, this means that the server can dectect the use of them. Any other mod other than CJB or zombe?, not possible.