You could amend your No Solicitors sign with "Includes any anyone proselytizing on behalf of a religious institution" so when they still knock you are under no obligation to be polite to them. They can read the sign. They know you don't want them to. So if they do it anyway, it's their own damn fault for the treatment they get.
Teenaged X-Files obsession + Bermuda Triangle episode + Self-led school research project = Atheist.