Group Policy in Win 2003

hi every 1, how can i set group policy in win2003 server to resrict the user from accessing the server, i m using win2003 std.server & xp pro as client in domain but some so called smart users are accessing the critical data on my server
how can i restrict them using group policies
or is there any tool or something like it to have log of the users activity orwhat about monitoring tool or software which can track users activity

Pls help me with this guys

thanks to every one there

Vjy