Can you prevent a client when logging a call from changing the default.
Yes.
Setup -> Tickets -> Options -> Client Priority Option
Set this value to None to prevent clients from setting a Priority.
Client Priority Option Indicates if all Clients or only Client Admins (both Location and Department) are permitted to set the Priority level of their Tickets. Otherwise, new Tickets will be assigned the Default Priority setting.
A setting of "All Clients" also enables the display of Ticket Priority in Client e-mail messages.