I've started reporting the worst offenders as spam. One service was e-mailing me once a week. After the 3rd e-mail I vowed never to use their product out of principle. After the 5th I'd had enough of the harassment.
The one I have in mind sent 4 emails in the same week, with a linear increase in bewilderment that I wasn't answering promptly beyond the first two (where I made it clear that I wanted to take my time).