When dealing with staff turnover, you won't want to delete as that can mess with the client history. What I recommend is the following:
1) Under Setup > Staff > click on the staff members that are no longer with the company. Select "No Services" under Performs. Once that staff member is listed as No for Performs, then we can adjust your billing if that was the provider that put you over to the next billing level. Just let us know when you have done this so we can review your billing.
2) Create a category under Setup > Staff called "no longer with the company" (or something like this). Then you can drag and drop the staff members you have deactivated under this category.
Please let us know if you need any further help with this.