It is perfectly acceptable not to reply to responses, but it's nice to provide a quick "Thanks but no thanks", especially it the guy comes across as decent.
 

WARNING: Some sites that offer "Free" listing of your ad, but a payment to respond to other advertisers, will enable you to register your ad for free but shortly after you have joined send you a message, supposedly from another user, saying they are interested in you. This message makes no reference to the content of your ad and is to entice you to pay the joining fee. They will also send a similar message if you don't renew your membership..