Because Flarum not give Admin to batch delete users,And there is no way to stop the unproven user registration;If you want delete all unconfirmed email users,you need login your MySQL server and execute the following command:
This commad will delete user from your database,This is no way to recover if you no backup,Please backup your database before execute it
delete from users where is_email_confirmed=0;
I don't think Flarum's registration logic makes sense, it shouldn't write user data to the database before completing user email verification. This could lead to malicious registration using other people's emails.