In response to Is It Legal To Block ‘Spam’ That You Know Is Not Spam?, Gnomie Ken writes:

As a practicing lawyer who hasn’t researched this, I can’t answer any of your questions; but let me make some suggestions if you mean to do some digging yourself.

Look into the definition of extortion as regards a fee for protecting your e-mail from being placed in the spam folder. (May not be privately actionable, but if the facts fit the definition you may be able to interest a prosecutor in writing a letter or even in…)

Research the cause of action for tortious interference with advantageous/contractual relations relations.

Think about an action by your organization as a representative of its members for breach of their respective contracts with the ISP.

Good luck.