Deleting leads, contacts, opportunities, and other records can be helpful for keeping SalesForce organized. However, this can also cause problems for your Marketing department. For example, deleting low quality leads can make calculating conversion rates difficult if not impossible. Additionally, it's not possible to remove access to delete records without also removing access to merge records.
For these (and other reasons) most companies prefer to reassign "deleted" records to a specific contact (typically someone in Marketing) rather than hard deleting them. We've developed a free app which allows you to setup specify who records will be assigned to when a user deletes them.