You can uncheck the allow discount check box on tab 2

BigLouie, do you mean the Allow Trade Discount check box?  If so, I tried that and it did not prevent additional discounts on a sales order.  Am I doing something wrong?

Out of the box,  you could set it so that a salesperson cannot due a price override.  Role Maintenace | Module Options | Sales Order |  Allow Price Override in S/O Data entries.   Of course that would restrict ALL price overrides.

With a user defined script you might be able to create some code to accomplish that only with items that are on sale.

Tom, thanks for the tip.  We can consider it if we are out of options.  

A script is probably the only way to prevent manual overrides to a lower value.

I'd set up a UDF for each item's minimum unit price, then script a lookup (column post validate / table pre-write) to enforce the minimum.