if you have removed the user via PHPMYADMIN and the user is “recreated” by him/herself, it sounds like the user you are trying to remove has access to manage your webhosting account. (I am assuming you know the user and have already checked for other malware)
I contacted the hosting provider to investigate this issue. Let’s see what they find.
Turns out, that the functions.php file had script, that recreated this admin.