Is a person sinful for their account remaining on a pornographic website after repentance, without using it, and not knowing how to delete it?

We ask Allah to grant you success in completing your repentance and to accept it from you. There is no sin upon you as long as you do not know how to cancel the account and have not added anyone or posted anything. If you find a way to cancel the account or change the password, then do so, as that closes the doors to what is forbidden and distances you from what reminds you of the sin. It is recommended for one who has repented to abandon the places where they sinned and the friends who helped them commit the sin, and to associate with people of goodness and righteousness.