It's very difficult to deal with family when it comes to religion. While they would cry and moan that you're being disrespectful if you simply state "I believe that you're wrong about gods and I don't subscribe to any of your religious beliefs" it's apparently okay for them to completely disregard your feelings and absolutely disrespect you by claiming that you need God or that you're going to hell or that you should give up your beliefs and just accept God or some other such drivel (and you'll notice this happen in general society all the time as well). You'll never get them to understand what jerks they're being so the only real option is to humor them when you have to and avoid the topic otherwise. If they can't even accept "I'd rather not discuss religion" then you can walk away (or not read their emails). Unfortunately, it necessarily falls to you to be the bigger person.
If it's a stranger, then "no thanks" and going about your business should really suffice.
As in the case of Cara, I also come from a religious family. In cases where the welfare of my soul is called into question, I like to point out that before King James re-wrote the bible to suit his needs, that hell is described as the furthest place from god, meaning one ceases to exist on any plain, so hell is infact what an athiest believes would happen when one dies. I have not yet had the pleasure of running into a mormon yet. Just try to remain civil, rise above it, let everyone else rant and rave.
It may be that we have to be more comfortable with our disbelief amongst all the family and peer pressure around us in order to cope with their preaching. If I mention I'm atheist I'm proud about it, I can defend the position well and often like the debates. In the end you can just say 'Well I appreciate your concern, but if you can't answer my questions I can't sign up', and you can even cover the 'just have faith' argument by asking why they don't just have faith in Krishna or some other god. Why just theirs? It's very rare that I won't feel like discussing it, more the other way round when I press them for answers!
*Insider knowledge* If you don't want Jehovah's Witnesses calling on your door, clearly tell them not to and by law they have to mark you down as a DNC (Do Not Call). You should get a few years peace before they test the water again to check if you've moved or changed your mind. Might work for the other religions too, I'm not sure.