How do we exclude existing customers from nurturing? Was told by Tech Support that it's not possibl

How do we prevent existing customers from receiving our marketing emails geared towards bringing in new business?

I just got off a call from support who told me this is a unique, customized item that I would have to pay for consulting services to achieve.  That none of our customers make this unique request.  I can't be the only customer who doesn't want to send existing customers marketing emails saying, “Please buy our product?”.  There’s got to be some way to do this. 

 Couple of scenarios –

I buy a list of contacts and import them into Marketing Ondemand Contacts.