Agree with him. The person is seeking to justify his sinful heart by spreading the blame around all of humanity, and at the same time inferring that the standard God requires is too high. Say, “That’s right; you are not perfect. You are a self-admitted liar, thief, etc., and you have to stand before a perfect God whose Law you have violated and give an account of your actions. What are you going to say? You can’t justify yourself. And you are right about no one being perfect. We have all sinned, and we are all heading for hell.”
The person’s words reveal a subtle form of self-righteousness, and you have to chop that out with the sharp axe of the Law. So cut deep and get it all out. You want to bring him to a point of acknowledging, “I have sinned against God.” His sin is personal. Study Paul’s use of the Law in Rom. 2:21–23. He stirred the conscience and made it personal. Nathan did the same thing with David when he said, “You are the man!” Study the opening verses of Psa. 51 to see how personal David knew his transgression was—knowledge that came from the prophet’s rebuke. His actions weren’t “imperfect”; he calls what he did “evil.”
Ephesians 4:13 … till we all come to the unity of the faith and of the knowledge of the Son of God, to a perfect man, to the measure of the stature of the fullness of Christ;
Ray Comfort's Daily Evidence 
Pingback: “How do you respond when someone says, ‘Well, no one is perfect’?” | Ray Comfort’s Daily Evidence | Christians Following Jesus
Ray, I’m an avid supporter of your teaching. For the last 6 years I’ve been hosting your seminar here in Phoenix AZ witih much resistance form several of the churches I’ve attended. But this messagae is still misssing the answer. When they say no one is perfect they are speaking of present & future sin, they admit their past sins. The next comment I get is,”So you’re telling me that NOW you never sin?” As I’ve said earlier, you and the Holy Spirit have given me the abiblity to share with everyone I encounter, so can you help me with this one?