How can I remove or hide the stocks app from Apple Watch?

You can delete the stocks app from your phone on iOS 10 - and that removes it from the watch. There isn't a way to have it on the phone but not on the watch.

There were currently no workarounds, jailbreaks or non-obvious ways hide and minimize the stocks app on the Apple Watch apps display on iOS 9 and associated watch OS. The rest of this answer applies if you cannot upgrade to iOS 10 and watchOS 3.

Consider making a long tail arrangement of apps placing the ones you don't want to see to the end of the line.

In practice, you'll see the few apps centered around the clock face central point of the graph.

p.s. still can’t remove the stocks app on watchOS 7.1 - the beatings will continue until morale improves.