Prevent unit of measure in SOE

Is it possible to prevent the use of an item's primary UoM at sales order entry?  We have an item whose primary is EA, and there are good reasons for that being the primary, but we now want to force customers to order in BX or CS units of measure.  The "exclude from SO" flag in the item unit of measure conversion table is supposed to work off the "to" uom, according to Oracle documentation, but it seems to work off the "from" uom.  Nothing seems to prevent the primary from being used.  Any ideas?  thx-dan