Is it possible for me to disconnect a computer that is physically attached to a linksys router without actually disconnecting the cat5 cable? i am the admin so i have the password for the router. i know how to kick off wireless connections, but is it possible to do this to a wired connection?
any help is appreciated!

Yes, MAC filtering surely do the trick. Just enter the MAC address of that machine and block it. Think most of the router comes with MAC filtering.
Cheers!