For me, I would just say, "okay!" and walk away as fast as possible. If they continue to follow me down and discuss more of that subject matter with me, I would bluntly point out that that statement needs some evidence and that extraordinary claims require extraordinary evidence.
I'd definitely say one of the nicer things like "thank you" as opposed to some comment that is probably only funny for other agnostics/atheists (e.g. "Lucifer loves you" ). As a former fundamentalist haring "Lucifer loves you" would have only made me think that there was spiritual warfrae going on and make me delve deeper into fundamentalism.
The agnostics/atheists I interacted with who never negatively or sarcastically referred to Christianity, and who were rather respectful, were IMO more instrumental in my deconversion than snarky atheists. Though I can't speak for all former Christians.
There is a church group that walks around a local park where people play Pokemon Go and kids play in the water fountain. They hand out bottles of water, then ask to pray for anyone who accepts the water. Of course, I never accept the water or the prayers... One of the guys has taken to trying to "save me" by always telling me "Jesus loves you, even when you don't love him" when I turn down the water. I started off just politely saying "ok." But, now I say "I wish you were more concerned with the water bottle waste you're creating than my eternal soul." Sometimes they try to offer my kids water, without talking to me and the kids always say "we have water bottles, because we don't like making trash!" Which is judgey and rude, but I let it slide for these pushy fuckers.
Back in 1973, I would walk home from school and everyday I was confronted by what we termed Jesus Freaks. Everyday they would tell me Jesus loves me. I was so shy at the time I would lower my head into my books and walk past even faster. Sometimes they made me even more uncomfortable by following me and quoting scrpture. I would never be rude, just scrurry by saying than you. Their "devotion" creeped me right the fuck out. Summer break came and I guess I grew up a little. 1974 School starts and those Jesus Freaks were out everyday.... in force... Jesus Loves you. One day I began to quip back. "Yeah, well he's going to have to wait in line with his money like everyone else." Jesus love you... "No he just wants to date me for my money like Chalie did" pretend to be upset and run. Jesus loves you.... "You tell him to stop, Javier has my heart" That was 7th grade. I like to think they helped form my thinking for the rest of my life.
I am immediately concerned about their mental health. Taking a delusion so far as to be obnoxiously pestering people on the street is a sign of extreme illness. (That's not to stigmatize mental illness, which I experience myself. Clearly, deep rooted delusion is the caise of great inner pain..) There's probably a manipulative millionaore running the operation. For that fucker, I feel no compassion at all.
I usually just walk on without saying anything. I don't feel affronted unless they persist.