Hiding "speaker gallery" in Skype For Business?

Just choose Gallery View or Content View. The setting is up in the upper-right corner of the meeting window.

I still haven't figured out how to stop Speaker View from being the default.

Choose "Pick a Layout" in the title bar and then Content view.

If you're referring to the annoying window hovering over the bottom right corner of the screen that shows the current speaker, I suggest pressing

Ctrl+Shift+K

which is the hotkey to 'Switch to content-only view'. This will remove the hovering window. When you know the people you're talking to, this window isn't necessary and blocks the content on screen.