How can I stop a "citation cartel" still active? How can I fight against a "citation cartel" (still active)?
They are manipulating citations (mostly through "coercive citation") from at least two journals. Two journal are used as "platforms" and two journals are receiving biased citations.   

Clearly, they are trying to boost one journal and also boost "selected" papers from researchers close to the editors of one the journals.
Awful!
Are there uninvolved editors you can report it to? Do you have enough evidence to contact Retraction Watch, which might take an interest? If the journal(s) in question belong to a scholarly society, perhaps a report to the board would suffice?

The ten-ton anvil would be contacting Thomson Reuters to ask for an investigation -- they'll boot journals they catch manipulating Journal Impact Factor out of the Impact Factor rankings altogether -- but I am not entirely sure how to accomplish this feat.