How can I release all existing database locks for all the sessions a specific user has started?
The easiest way probably is to log that user out. Otherwise, you can find out which locks are out there by typing onstat -k. Find the address of the owner of the lock under the "owner" column.
Then run an onstat -u and look for the address of your culprit owner. Look under the "sessid" column to get the session id for the user holding the lock. Then do an onmode -z with the session id, which will kill that user's session (and zap the lock).