First of all, as a marketer, you will be faced with negativity in promoting just about any product, so you must be prepared for that.
Your responce should'nt be too lengthy, as you can't force SFI on your prospects otherwise you turn out to be rude or harsh, you could just conclude by saying maybe some other times he or she could give it a thought, but do not leave without leaving you card or url with him/her.