Under IIS 6 you can remove user sessions under the FTP tab. Under IIS 5 im not aware you can end user sessions, but I cannot test it atm as I ont have IIS 5 installed.
This gives you the general information about the ftp tab in IIS6
http://technet.microsoft.com/en-us/library/cc771047%28WS.10%29.aspxInformation on how to disc a users ftp session.
http://technet.microsoft.com/en-us/library/cc753814%28WS.10%29.aspx