Prevent customer from changing incident priority when creating a new incident

I had to create several dummy tickets to find out the logic behind how the priority is assigned based on the selection the customer makes in the customer portal.  Here is how what we have setup gets assigned.  We have 8 priority levels.

P1: Critical (Emergency) - 1 hour

P2: High - 4 hours

P3: Normal - 8 hours

P4: Low - 24 hours

P5: Informational - 40 hours

P6: client Service - 80 hours

P7: Compliance - 100 hours

P8: Project - 480 hours

Here is the matrix for how the selections affect the priority assigned.  We have some request types setup to be assigned a certain priority, such as report requests.  See table in pic attached.

Hi Marsha,
At this time it is not possible to restrict customers from limiting the priority level on new incidents.  I apologize if this is an inconvenience to your organization.

Thanks for the information. So it's not arbitrary but it is highly illogical. How do we make it stop? This is senseless.

Perhaps it makes more sense for you to not allow us to create custom priorities, since you essentially determine the priority on our behalf, as seen in the chart above. The other solution is to allow customers to see our priority labels and choose from that list. For instance, if a customer chooses that this effects a single person and it's non-urgent, that relates a to priority that we have labeled as "Quick Fix." I am now reprioritizing thousands of incidents because we can't have customers driving our priorities with the SLA's assigned. Essentially, I've had to get rid of the Request sections and write off priorities one through five. So, now my top priority number is priority six. And I can't do this all as a mass action because my old priority sixes will get mixed in with my new priority sixes, which are actually my top most priority items. I have no nice words about the product right now.