Cleaning the temporary users place

Hello everybody !
I installed a new PunBB. 1.4.4, sqlite3, php7.

Users begin to register. Some meet issues with the registration.

After clicking on the link to validate their email address, they cannot even enter the password for the fist time and they are welcomed with an error message. Maybe they did not notice that their user name was case sensitive, I don't know. But they now are stuck in a kind of in-between, a no-mans land, not registered completely, but not able to try a new "first mail validation", since it says that the ticket is not valid anymore. And they cannot start again the registration, since an error message says that the email already exist.

I would like to help by using the add-member extension. But, I can't because "email already exists", even if the member is not truly registered.

How can I clean the temporary files, I mean deleting the temporary pending profiles ?

Thanks in advance.