If you'd like to hide non-applicable workflows from being seen by users, it is said that you can hide a custom you can remove a tag named 'Workflow:WCM' to control the workflow to display or to hide. No so true, at least not for CQ/WEM 5.6.1. My experiments shows that to have a workflow show in sidekick, the workflow page/node must either have no tag at all, or be tagged with 'Workflow:WCM."
Should you want to hide a workflow from the sidekick, the trick is to tag it with anything else, for example, "Workflow:Misc":
Remember to save (step #3) when you're done.