In the past, we outlined a method to block sign-up form spam by changing the encoding type of your sign-up form. That method may be found here.
This method will not work, however, in cases where the spambot isn't coming through your web site's form but rather by avoiding a form alltogether and programatically doing a FORM POST to our form processor file, http://www.jangomail.com/APPLICATION/PGSF2.asp or http://www.jangomail.com/APPLICATION/PGSF2ENC.asp.
To defend against this new technique for propagating sign-up form spam, you can now specify an exclusive Referrer URL from where the signup form must originate. You can now assign a specific exclusive Referrer URL to a particular JangoMail Group. Then, if a form signup comes from a destination other than the exclusive Referrer URL you have designated, the signup will not be processed.
Currently there is no interface within JangoMail for you to be able to set an exclusive Referrer URL yourself for an e-mail Group. Currently, our support team must do this for you administratively should you wish to take advantage of this setting.