How to disconnect abusive ftp user

I am operating a video ftp site. As I logged in as root, I used ftpwho and found that a users has opened five connections.

Is there a way to disconnect this guy?

Also, where can I find good site/tutorial for learning networking commands in linux, such as listing all the connections/ports/... or acting on a connection.

Thanks,

Peter

 

 

 

 

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