Hi Emmanuel. Are you using captcha for your registration form? It is designed to stop spammer sign-ups. https://docs.moodle.org/28/en/Email-based_self-registration#Enable_reCAPTCHA_element
That page mentions the spam cleaner report, which may help to fix the current mess.
There is also the settings to delete unconfirmed and incomplete users after a certain number of days. https://docs.moodle.org/28/en/Cleanup#Delete_unconfirmed_users_after