To use a backdoor script it would have to give him access to the console to unban him.
To have access to the console he would need SA.
To have SA he would still need to have that when you banned him (You cant ban SA so yeah he would have been de-admined)
To have not been banned he would have needed a backdoor script
type /read it over again
With best chance one of your admins is unbanning him to screw with you.