Prevent Users from Selecting 2 Values in an Option Set (or remove them!)

I have an option set with about 6 values (customer, prospect, lead, competitor, etc). I need to prevent users from selecting 2 of the values in the option set as they are set through a business process flow behind the scenes (specifically, customer and prospect). How can either

1) Dynamically remove these from the optionset list values when the form loads, or 

2) Prevent the users from changing the values to either of those?