You can do this using Alternative Price Lists (APL) in Inventory. Create an APL using the customer code as the APL code, assign certain items to that APL, and assign the APL code to that customer in the Price List field of customer maintenance. Then when you try to sell that customer an item not assigned in the APL, you will get a message. The message will be either a warning message or a lockout message, depending on a setting in company profile.

Is this what you had in mind?

Steve