No i don't believe you can restrict the Client user from going into History on their Client portal and changing that value.
However, if you let the user choose the Priority level of the ticket instead of creating a custom field for it, i don't think the Clients can modify that field once the ticket has been created. That might be the simplest approach.
There is another option as well if you wish not let the Client user select the actual Priority of the ticket when they submit it. That is to:
- Create a custom field like you did (ZZZ-Priority2 ... but i would name it perhaps Impact)
- Make sure that you have Priority levels that match those options that you put on the custom field (like "normal", "urgent", "blocking"). THere could be other Priority levels as well if needed.
- Create an Action Rule which says, essentially: On ticket creation only, if the submitter chooses "Urgent" for the Impact field, set the Priority for the ticket to Urgent. Because that only happens on ticket creation only, the user could go back and change the Impact field later but it would not affect the actual Priority field. In essence, it allows the Client to be able to have their say about the current status, but it would be up to the Tech to change the Priority if it really needed to be changed.