Can I only use "Customers" when entering sales orders?

I'm trying to restrict our employees to only use actual customers when entering sales orders.  The problem is that when you search for customers when entering a sales order or invoice, it searches through leads and prospects also.  Is there a way to stop this?

I would also look at doing away with leads and prospects completely, but when I try and disable sales force automation it says that I can't use commissions either, which I need.  Thanks for any help!