How to complain about an abusive editor? It is well known in my field that a particular good journal has an editor-in-chief that frequently asks authors to cite a particular (largely unrelated) paper whenever he has the chance.

Looking at google scholars citations of this particular paper, you can notice that it has a large amount of citations from this particular journal. Reading the papers you can almost see the struggle of the authors trying to cite an unrelated paper. 

This has been going on for years.

What can I do to try to stop this? I frequently avoid submitting to this journal due to this issue.
If you want it to stop, the best way to address the issue is to address it with the editor himself. That's not a guarantee that it will be addressed, it could very well be the case that the editor is aware of how his requests are perceived by the most people in the field, and is choosing to run the journal this way anyways.

You mentioned that you avoid submitting to this journal. If you want to lower the prestige of the journal, you could publicize the unprofessional practices of the editor, and encourage your colleagues to submit to better, more professional journals instead. As MxNx mentioned in the comments, there are many good journals they could submit to instead. If many people in your field stop submitting to this journal, chances are it will fall in prestige and possibly fail eventually, unless the practices change.