Its not a plugin. In the admin panel Settings->User->Default User Settings
How Do We Disable Daily Digests?
Daily Digest emails are working and work pretty well. Only one little snag, we can't turn them off and we have users who are annoyed by the uninvited intrusion, for obvious reasons. Is there any way that we can disable them or control them? Controlling by user would be ideal, but short of that, we need a way to cut them off completely. Any ideas?
Apparently it should be sorted in the next update/version.