Before I had a "no proselytizing" sign on display, my responses tended to be civil but firm and focused on ending the interaction as quickly as possible. I've always viewed proselytizing as an unwelcome invasion of my privacy. Unlike several of my readers who have commented on previous posts on this subject, I do not have any interest in interacting with the sort of Christians who do this a moment longer than necessary. The last thing I want to do is reward their intrusion.
Since I put the sign up, I have given myself permission to speak my mind far more freely. You see, a Christian who has the nerve to ring my bell now has made his or her way past both a "no soliciting sign" and a "no proselytizing" sign, willfully ignoring them to interrupt me. Depending on what I was doing prior to the interruption, such an individual may simply have the door closed in his or her face or perhaps be treated to a delightful symphony of profanity.
Maybe I should be more concerned about shaping impressions of atheists and the like. I'll do that in my professional life. I'll do that in my social life. Hell, I might even do that here on occasion. But I'm not going to do it when someone intrudes on the limited free time I have to enjoy in my own home. No, if you insist on peddling that crap to my door, you're going to hear what I really think about it. I think that's fair.