Disallow Users to Change Price List

My client runs Sage 300 Premium and is in the business of selling Juices. Now the client has set up several price lists for different customer groups e.g. Distributors have the most favourable price list and retail shops have the highest prices.

When doing order entry the data entry staff when selling to retailer are able to change the price list code for the customer to distributor hense selling at a lower price than allowed.

Is it posssible to lock the field for price list code such that once a staff selects a customer to sell to, the price list code associated with that customer remains fixed.

All suggestions will be highly appreciated