Using the autoresponder add on you can setup your followup (one or more) addressing it to a list. When a product is purchased you should remove the customer from the targeted list.
If you use woocommerce when have a set of filter you can configure to accomplish the behavior above.
Alternatively you need a bit of custom code which is able to set the list_N to zero in the subscriber table for the customer which needs to be removed from the targeted list.
Stefano.