If you ever want to annoy an atheist, become one. Or at least, become like one. You've seen it many times before. You log onto Facebook, you see a religious post, you see an atheist making silly arguments against God. What should you do here? You turn it around. You ask the atheist to prove that there is actually a living person behind that keyboard. Then you raise your standard of evidence to their level.
The design in nature is so obvious that Richard Dawkins wrote, in The Blind Watchmaker, that "Biology is the study of complicated things that give the appearance of having been designed for a purpose". But of course, atheists, by definition, do not believe that biology was designed. Similarly, there is a story about Isaac Newton being unable to convince his atheistic friend that nobody made the model solar system on his desk, and yet this same atheist continued to believe the solar system, of which it was a poor imitation, was not. Long story short, atheists do everything they possibly can to deny the existence of a Creator despite all the evidence pointing to His existence.
So, you have license to do the same. No matter how irrational you feel (and you will feel irrational, because you are being deliberately irrational), do not let the atheist convince you that their comments were written by a real person intelligently tapping on a keyboard. Stick firmly to your conclusion that somewhere out there, a grenade exploded in a junkyard, resulting in a fully functional computer facility, complete with its own powerhouse fueled by solar, wind, and water power, and that a computer within it eventually developed an algorithm so advanced that it is able to create an entire atheist Facebook profile, complete with the capability of writing seemingly intelligently designed comments.
You could even go so far as to pick up on their spelling mistakes as proof. If they make strange, seemingly out of place comments, you can use that to further your case, as an intelligent being would presumably be better at making a response that is actually related to the previous claim. They might offer to send you a video, to which you can reply by linking them to a clip from Jurassic Park or some other movie with an impossible scenario (the crazier the clip, the less crazy it becomes to say a computer can easily CGI a video).
Set an experiment up like this and have some fun. Using an atheist's own logic against them to prove their comments are not intelligently designed ultimately shows them how frustrating they are to deal with. There's no logic in denying the appearance of design that is so obvious, even the great Richard Dawkins cannot deny it exists. But he tries. They all try. But through this experiment, you show them the absurdity of trying.
It can be hoped that seeing the absurdity of trying to deny God will convince them to start seeking Him. This is where things get tricky. As much as nature clearly reveals that there is A god, it does a terrible job at showing us which God. The men of Athens, whom Paul addressed in Acts 17, even went so far as to erect an altar "TO THE UNKNOWN GOD" (v23). From here, take them to the scriptures, which describe the only God in all of philosophy to ever enter into history and show Himself. Jesus Christ, the Son of God, came to the earth as a man, lived a perfect life, died on a cross and rose again, a claim that no other god can claim to have done. Thus, we can have complete faith in the fact that Jesus is the real God. Through that faith, we can be forgiven for all of our sins, and be reconciled to Him for all eternity.