Thanks to Tony for this!
Obviously, you can use the Terminal Services Manager (now the Remote Desktop Services Manager) to find people and log them off. This is a command line way to do the same thing. Easier, possibly, and doesn't (to my knowledge) require the Manager pack be downloaded & installed.
To list users:
query session /server:yourservernamehere
To kick off users:
RESET SESSION 1 /SERVER:yourservernamehere
rwinsta /SERVER:yourservernamehere 1
(replace 1 with whatever the ID is in the results from the QUERY)