I didn't know there were cheats for minecraft lol.... help?

Lol so I'm still new to the server business and all that... Well I'm going to be hosting a few PVP events on my server sometime in the near future, and just recently one of my friends found mods that work on multiplayer too! I knew there were mods for minecraft, however I tried a few on multiplayer and they wouldn't work so I thought mods not relating to craftbukkit couldn't work on Multiplayer, well how wrong I was lol, So, now I have a friend who has advantages over other players, And as I did more research, I even found an Xray mod that people said it worked on multiplayer, so... how exactly do I block these 3rd party mods? so far the only thing he's using is a minimap plugin and some plugin that allows him to go over 64 stacks... any suggestions would help.

and if your wondering why I simply don't tell him to use those mods, well lol, I live no where near him, he's just someone I know over Minecraft, and he's the type to say he's not cheating but really he is lol. and I have another friend who as soon as he finds out about these things he'll download them. so again, any suggestions would be helpful

 

You can use Orebfuscator to block Xray, and NoCheat+/AntiCheat to block flying/etc.

See various threads on this subject:
https://www.google.com/search?q=anticheat+plugins+site:bukkit.org