From the Melapress blog:
Terminating all WordPress logged in users sessions from the database
There might be edge cases in which the plugin cannot delete all the logged in sessions. For example, if a website has more than 100,000 logged in sessions. In such case most probably the plugin’s process times out.
If as such happens, then we’d recommend you to delete all the logged in sessions on your WordPress directly from the database. You can delete the sessions by running the following SQL query on your WordPress database:
DELETE FROM wp_usermeta WHERE meta_key='session_tokens';
Note that the wp_ table prefix might be different on your database, so update the query accordingly.
