Our agency always spends more time in Analytics at the beginning of the month than any other time. While we were in Google Analytics this month we noticed that the amount of spam from referring traffic just continues to grow :(.
The other way is if you own your own server. If you do there are a couple options to achieve this:
#1. Obtain the IP address(es) for each domain and block within the firewall.
#2. Setup an Exim System Filter File to block all exim traffic server wide to these domains. See the link below for further detail: