I've found that most of the Christians I know are extremely good hearted people who want to save you from their terrible god.
Coincidentally, I was reminded of that today when I went to church on christmas morning. Someone mentioned to me "once you know how terrible hell is, you can't help but to go out and try to save as many people as possible."
Ravi Zacharias describes a common Christian view of hell here:
http://www.youtube.com/watch?v=Rw4YkpzEVg4You can get the gist of it in the first 2 and a half minutes.
I know people who spend almost every waking moment (outside of work) to "winning souls".
To many evangelical Christians, it's not just that they are following god's command to spread the Word and become "fishers of men". There's an urgency as well. They really think they are saving people from the most horrible eternal torment imaginable. Many think they can't even imagine it.
And they keep asking and asking even after you reject them because even after you reject them, it doesn't end there. After you reject them they will pray for you. They will pray to their terrible god to help open your heart to him. And so they keep on asking in case their god has answered their prayers.
These people cannot be reasoned with. The best thing to do is to just politely ask them not to contact you again and don't pick up their calls. Tell them you have heard what they have to say and if you want to find out more, you'll call them instead.