Wow, that is definitely a little excessive. I try to average around 2/month (total), but that is split over probably 5-10 journals I review regularly (with a few others here-and-there). Refusing the request to be on the editorial board is a little nuts for the amount of work you are doing for them. I'm also a little surprised. What do they have to lose other than two lines of print? Its not like there is some hard limit on how many folks you can have on the editorial board or that these folks are usually compensated in any meaningful way.
I'd just refuse to review for them moving forward, or at least review very selectively. There is one journal *cough* Children and Youth Services Review *cough* that after an incredibly negative experience publishing in and the editor proving himself to be an incompetent jackass, I now take great pleasure in responding to each review request with "Based on my experiences with this journal, I do not consider this to be a legitimate outlet and thus do not want to support it by doing reviews." Oddly enough, they continued to send me review requests despite those comments though they finally seem to have stopped.
