Is it possible to have the Custom selection for Price level be removed from the transaction for particular user/role?
You can remove the field entirely from a custom sales order form. Then you can make particular user/role use that form only.
The drawback here is that you will rely entirely on the correct price level being assigned on the customer level. But if you are trying to remove custom pricing, I assume it is so a user can’t change the price so it should be fine.