The way I've done it is to issue commands directly to the database to remove the sessions for users you want removed. Assuming you're using a SQLITE database, the commands would be:
1. Launch the sqlite command-line client:
sqlite /path/to/your/project/db/trac.db
2. Remove users:
sqlite> delete from session where sid = '<username1>';
sqlite> delete from session where sid = '<username2>';
sqlite> delete from session where sid = '<username3>';
Quinn