Hello Vince,
Here's my answer. If you wnat to add a person whom you don't want as a customer, you can add him as a lead by clicking on tab Leads -> New lead. So in this way that lead won't reflect in your customers. However, you can convert your lead to Customer anytime you want. We have a great article on Leads in our Helpcenter, herewith providing you the LINK for the same to give you more idea about leads.
Hope this help.
-Inder