Unless your staff is actually deleting order lines or orders, preventing deletion (or 'unlink') using access rights wont do anything. The cancel action is, from the ORM's point of view, just another update (or 'write') operation. You will have to catch the cancellation of orders in the actual method that is providing that functionality, or, modify the 'write' behavior to catch these specific updates.
Alternatively - but that is applicable to Enterprise setups only - one could try to work it out using approval workflows as outlined here: https://www.odoo.com/slides/slide/10494