How other than netting can you keep frogs out of your hot tub?

You can't keep them out. Just klll them. When you open the cover, have 4 paper towels in your hand kind of wadded up a bit so you don't touch them. They won't move, so you need to move fast. Grab them with the paper towels in your hand, wrap the towels up around them and sqeeze hard. Kill them. If you don't they will come back. Don't get attached. They're nothing but vermin, carrying diseases. They'll crap in your tub. Kill every one you find. When it freezes, they will be gone.