I have two files, one of customers who have already purchased and another one of prospective customers. When mailing the prospects how can I suppress the names in the other file who've already bought from me?
Normally you would have a custom field for 'type' - and fill it with either customer or prospect. Then use segmenting to just send to the group you want.
If a prospect becomes a customer, update their record and they'll automatically appear in the customer segment.