Can you (anyone?) poll a w2k3 -server to check if an user is already logged on?

Say you have an user john logged on to a w2k3 server. Can you (anyone) poll the server to check if that user is already logged on?

To use that in a script to block a new logon, if the user is already logged on.

As Microsoft doesn't this support out of the box (there is an utility which can do it, but that costs money).

Something like:

if %logonname% in %listofloggedonusers% than logoff

or

if %logonname% in %listofloggedonusers% than deny logon

(These example won't work, because these commands don't exist. But you get the idea.)