What does your religion say about those who have either accepted the invitation or were born into the religion in the first instance, but eventually decide its not for them and want to follow another school of thought? Or not follow anything at all?
Do you invite him (or her) back?
Do you know name of the person who firmly believed and knew in his heart that the message Prophet Muhammad (PBUH) has brought is true, but still won't accept it?
What are your religions views on others who also believe in inviting you to their side by conveying their message to you? Is it a quid pro quo? Do you listen and then decide to move or politely refuse depending on your level of conviction?
Explain the manner of invitation, there is always a difference between exploiting someone because of his helplessness and simply letting him know what you want to convey to him without any exploitation or material reward involved.