How about just "Thanks," since they are sending you friendly, positive thoughts, well proven to be effective, as shown in the placebo affect.

Thank you. You too. It was a gesture of good will, after all.

I agree with birdingnut. They are trying to be kind, why get snappy with that? Simply saying thank you and/or have a nice day too is best. By being snappy you make yourself look like an angry crusader.

"You have a blissed day, too."

I'm here,the universe has blessed m! Thank you for noticing!

I don't bother replying in a negative way; I realize that for believers that's their way of saying "have a good day"

Depending on context there are times I'll bring up my non-belief but thank them anyway, but most times I just say l tell them to have a good day as well

They mean well. I have no problem with it. I don't say God Bless you when someone sneezes. I say bless you. There is a difference.

As several have already typed, a simple thanks and a smile go a long way. Friendliness has nothing to do with religion. I can call some of my friends shithead, but they know I do so with a smile.

i would say, "as the universe pleases" and be done with it.

you: "Blessed by whom?"
her: "By God of course!" (Assuming the Christian god.)
you: "Ah... vanity. That breaks a commandment."

The implied statement actually is along the lines of "may God bless you." The thing is that there's no reason to say this to the person one is wishing God's blessings upon, as it is essentially a prayer. It is done purely for vanity's sake, i.e. taking the Lord's name in vain.

I need condoms

My response to all such similar well-wishers is, "Thanks, I'm workin' on it!"
But I also like, "May the Force be with you".

I’m in Mississippi so I get it all the time sometimes I say nothing like in long lines .. but if I have time while I’m gathering my purse or whatever,..
I normally tell them, “try have a nice day cause we’re not all Christians.”
sometimes I say “Santa clause to you too.”
But if it’s a cashier I’ve already politely told I’m not Christian and they say , but god loves you anyway.
I normally respond with “all hail Satan”
Not that I believe in a xtian form of Satan at all, but Satan as a symbol for rebellious people.. and for shock value..

Each case is different.. if it’s an old person I just smile ..
feel it out and have fun..
Other things I say in return are ;
“the force be with you.”
“ have a magical day”
“By Odins beard you too.”
“Seven blessings “(GOT)
“Doesn’t compute”
“ pixi dust”
“Tardis appreciation day”

Anything odd back..

I just take it as it is (in my opinion) meant -- to be nice. Their beliefs, being kind. No more.

Does it upset you?

Namaste.

I say 'I's better than a cursed day'.

I just smile and nod my head. They mean well.

And may the flying spaghetti monster bless you with his noodly apendage. ?

I think a snappy comeback is not a good idea. Religious people already have negative stereotypes about us nonbelievers and this would just reinforce that.

I generally just ignore them. Just nod & walk away.

Go to hell

"Thank you. I'm an atheist but I appreciate your sentiment."

"May your life prosper in the four blessings
of old age, beauty, happiness and strength"
--Secular Buddhist Blessing

“Have a blessed day” is definitely a put down. Whether the holier than thou, poorly educated, christo-fascist recognizes this or not. It assumes a great deal and hijacks the societal secularism of “Have a pleasant, nice, great, wonderful, etc day” by appropriating your day into their kook religion.
This point is easily proven by them being placed on the receiving end of a Satanist cheerfully waving them goodbye while saying “Have a sacrificial day!”. How could they possibly take offense? (Especially when their manual is full of witches, ghosts, demons, and plenty of sacrifices ordered by their god).