This website is for Christians and those interested in Christianity (the site rules specifically say so). Here is rule #2:
2. No conduct that is offensive or counterproductive to fellowship.
We like to welcome all to Christian Chat, but if anyone is not here for fellowship (or for wanting to know about Christianity), but simply for disrupting fellowship, offending people, whatever, then that person is not welcome.
I don't expect atheists to suddenly realize how much Jesus loves them; the Bible specifically says that unless God reveals it to them, they won't see. It often takes awhile (if at all in this life) to come to the point where the blood of Jesus being shed specifically for one's own sins really registers in one's soul. However, though we shouldn't hold it against people if they don't believe as we do, to allow them to blaspheme our Savior Jesus on a Christian site would be wrong. After all the pain He took for us on the cross, the least we can do is keep those who aren't yet interested in hearing of Him (but are instead here to start fights and argue) off of the site.