oscommerce manufacturer tweak

I am using OSCOMMERCE php shopping cart I want to PREVENT customers from adding items to the shopping cart if they are from different MANUFACTURERS e.g. if i select a HARD DISK from SEAGATE and add this to the cart and then a MOTHERBOARD from SEAGATE this will add also BUT if i then add a MONITOR or HARD DRIVE etc from FUJITSU for example i want an error message to popup saying " SORRY YOU ARE TRYING TO ORDER ITEMS FROM DIFFERENT MANUFACTURERS ! THIS IS NOT ALLOWED, Please only order from ONE MANUFACTURER" OR TO ACHIEVE THE SAME EFFECT IS THERE ANYWAY TO RESTRICT ITEMS TO ONE CATEGORY i.e if they select an item in 1 parent category (e.g SEAGATE) then they must choose all items in that (SEAGATE)category PERSONALLY I WANT YOU TO TRY TO SET THIS UP ON THE MANUFACTURER FILTER