Hi Manu,
several things to try, if the user of session 99 can get to TCL in another session they should be able to LO - 99 and logout that rogue session.
from within U2 Extensible ADMIN > LOCKS try Clear Locks for that PID (99) session, from within the File Lock Waiters TAB, which you've probably already tried
Have you looked for any Dead Locks
And finally and probably completely unapproved and am sure one of the Rocketeers (John Jenkins ) will offer the real approved method, if you go into Windows Task Manager and look for the TL_SERVER.exe with the PID = the session umber (99) and End Task, that will free the windows activity. and you should be able to close that session in U2 Extensible ADMIN > USERS
I hope this helps, but again all at your own risk !!!!
Thanks
Andy