How do you deal with "fake" customers? (those who are not serious buyers)?

Try closing technique and if he is not interested then he will directly or indirectly express his unwillingness. so you can ask him why he feels so and make him to give commitment. 

Sell to them in the same way you would a real customer but work hard to educate and entice them to open the possibility of becoming a real customer.

Promote the required  product to the fullest and later give them other options