cannot remove anything is a feature, it prevents people from changing history (imagine being able to undo all your outgoing payments)

And the reason you have to solve 5000 sudoku is by proving you have a limited resource, in this case processing power (and proof of stake makes it the money you have and skip the 5000 sudokus). Because if you are identifying people by a user account, there's nothing stopping 1 person from creating 20 billion accounts.