How to completely remove a plugin?

hello,

My friend has installed a plugin then activated it. To work with it he had to enter the registration email. He entered a wrong email. I corrected it and tried to register again, but the plugin still says “wrong email”.

I’ve deactivated and reactivated the plugin to try to overcome this but still same problem.

When I delete the WordPress database tables related to this plugin, I can’t re-install it and always says error. I’ve uploaded the backed up tables and emptied the tables so as to get rid of that wrong email but still can’t install it.

I even tried to install the plugin as a trial user not registered but still get same problem.

Any help on how to purge completely my system from such a plugin so as to re-install it?

Thanks