Recently we had to spend some time on fighting email spammers, who used “Share” function of Checkvist to send emails with hand-crafted “share note”.
The summary of what worked for us is here: http://blog.checkvist.com/post/166940442968/how-to-stop-spammers-and-not-impair-the-good
What did not work:
- blocking spammer IPs (they have plenty resources, really)
- adding limitations by a number of sent invitations (they created different lists, different accounts, removed and re-created invitations)
- avoiding sending many emails to a particular domain (qq.com is the famous one).
Maybe our experience will be useful to some of the people here