For me, this works. When I have a prevent_item_use power which prevents elytra from being used, right-clicking it won't equip it anymore. That's all a prevent_item_use power is supposed to do - prevent you from using the right-click functionality of an item. If you want to actually prevent elytra from being equipped through other means as well, you'd have to add a restrict_armor power and prevent elytra in the chest slot.

If you expected different behaviour than what I just described, feel free to comment on this issue and let me reopen it. Otherwise I'm going to close it, assuming you had different expectations from the power type.

I was hoping it would prevent elytra from opening while it's equipped. I wanted to put a doublejump power on spacebar, but if the player was wearing elytra it opens at the same time making it quite wonky. The plan was to make elytra only openable while doublejump isn't available.

Actually now that I think about it, a more generalised feature of conditionally blocking vanilla inputs or replacing them with an action would be quite nice.