How can I restrict users to set their ticket priority?

'Priority' plays an important role for support teams in streamlining the ticket flow. It speaks a lot about a user's concern and hence Ticket 'Priority' field now comes enabled by default where the customers can define the 'Priority' while submitting a new ticket from Client Support Center.

If you do not require this in your support process, you can simply disable ticket priority selection from a direct setting in Admin interface:

For this, go to Admin Control Panel  and click on 'Templates' on the left navigation bar, now click on 'Groups' to see the listing and choose the 'Group' you are configuring for.  Under the 'Settings : Tickets' tab,  disable the option next to 'User can select a ticket priority'. 

After this, your customers will no longer get the option to set the Priority while submitting tickets from the client support Center.

Note: Customers could still be able to change their ticket Priority after the ticket submission from under the 'My Tickets' section on Support Center depending on the configuration at: Admin CP > Users > Manage User Groups  > choose the Group > 'Permissions' > expand 'Tickets' category > check next to 'Can change own ticket's status and priority' (which is enabled by default).