You can setup a SQL query with the SQL manager within the prestashop.
Something like:
Then I went into the table directly (need phpMyAdmin)
ps_request_sql
and i simply modified the SQL statement to DELETE... (need phpMyAdmin because PrestaShop does not allow to directly add a delete query)
You can execute the query within presta shop now anytime you need it.
(It will execute and kinda show a crash message.... However it WILL run the query... simply return to your shop and all non active users are deleted.
BEWARE... with this method, depending what SQL query you write... you can destroy your shop...