What reasonable steps can I take to report misbehavior by publishers to the wider research community?
If the matter concerns a mathematical journal, you can submit your case to the European Mathematical Society ethics committee

Note that even if this committee has been established by the EMS, they should consider non-European issues as well as issues from European publisher, journals and authors.

Of course, their power is somewhat limited but given their status, they could have some impact by simply issuing a public statement.