As an alternative to using memos, check out the Purchase Control option in Sales Order. This is turned on in Sales Order Options > Line Entry > Purchase Control (third frame down on the left side of the panel). Purchase Control allows you to exclude specific items or product lines from sale to a customer. Once the option is enabled then go to Purchase Control Maintenance (found in the Setup menu of S/O).

Optionally it could be set to enter by state code instead of customer and also to exclude all items and product lines by default so then a list of items or product lines that can be sold are entered.

I will definitely check this out. This could help us out a lot.