You could use ps to see the list of processes and filter it for the username. For example, if someone logs on as joeblogs, you could use:
ps aux | grep '^joeblogs'
You could then use the kill command to kill their shell and any other processes you didn't want running. This requires root access.
A better approach would be to stop them from logging on in the first place or at least limiting what they can run. Who are these remote users?