my response usually depends on their attitude and how busy I am.
first off I say "this is a pagan household" and indicate towards the large pentagram by the front door
sometimes that's enough.
what happens next depends on their attitude to me.
if they're nice enough then I'm nice back and explain that I have a vast knowledge of the bible due to it having been part of my life for more than 25 years in the past and simply can't believe the same as them and that I respect their reasons and the fact that they care enough about total strangers to do what they're doing and have a nice afternoon and goodbye. I know they're there out of a sense of love or concern, and I thank them for it even though I think they're in error. To do otherwise would be like telling someone who stopped to see if I was ok if I fell in the street to "<bleeep> off" - there's just no need to be rude to them.
if they are rude or pushy, I push back. And since I've read their bible and studied it and know far too much about it to be convinced of it's "truth" and can speak christianese fluently... the more they push the more I ask them for answers they can't give about the inconsistencies in the bible they're basing their beliefs on. And if they beleive the bible because it tells them it's the truth well... to me that's a circular argument with no basis. And eventually they either see that I do know what I'm talking about and have made my decision as a well-informed person and go away... or I tell them to go away and look for answers to the questions I've asked and come back when they have them ... and they go away (none has come back with answers yet)
the ones who tell me I worship the devil... well if I did, I'm sure I'd know about it. My gods don't demand worship, just respect. I don't believe in a devil, so nope, can't worship something that doesn't exist. And I'm not headed to hell... the only hell that exists is in norway, apparently it's a nice place but since I don't have a passport I'm not headed there.
the ones who really get me angry are the hypocrites who don't know their own religion. telling my christian friend that the celtic cross necklace she wears is a pagan symbol (uh.. no. it's a christian one, you moron. and that was a pastor!). telling me I'm closed-minded (hello pot, this is the kettle calling...)
the worst one - told me I was a bad mother raising my baby for Satan. you do NOT come uninvited into our driveway, disturb my morning, tell me my son is adorable and then criticise my parenting for trying to raise him to respect ALL religions and make his own choice of path when he's older. I don't go to her house around and tell her that she is raising her children to be judgemental <bleeeps> in the name of Thor. She doesn't believe in Thor, and I don't believe in Satan. And neither of us know a thing about each other's parenting style.
all that aside, I don't choose which charities I support by "whoever harasses me will do", I chose by my own conscience and based off my own research. So I'm for sure not going to chose my beliefs for religion or politics by "well these people knocked on my door once" either!