Q - I thought that under User Global Security there is a flag you can set so the user cannot change the unit price on the Order Entry Line Item screen and just let the system assign the calculated unit price. But I can't find such a flag.  Is there a flag that does this?

A - There's no such a flag. Typically, we suggest implementing this as an internal procedure. Then you can audit whether users violate this procedure by comparing the following two columns:
    LINE_ITM_UNIT_PRICE
    LINE_ITM_PRICE_ORG
Where LINE_ITM_UNIT_PRICE is the current unit price in the Order Line Item screen and LINE_ITM_PRICE_ORG is the original calculated unit price before it is overridden, if that ever happens.

There are a lot of reason why the unit price may need to be overridden. What we had found is this tend to be a more effective than totally stopping user from changing the unit price.