You can possibly lock your opponent's monster zone if he/she has no tribute monsters. Use Floodgate Trap Hole when your opponent Summons a Monster. Then disable the 2 remaining zones by using Ojama Knight's effect. You can then wait to get the cards that you needed or burn your opponent.
The text "monster(s)" on Floodgate Trap Hole means that it can affect multiple cards. See these screenshots below for example.
You can't use Floodgate Trap Hole in response to an effect activation that would summon a monster. The summon has to be an inherent summon, which you can read about more here: http://yugioh.wikia.com/wiki/Special_Summon
Are you sure?
I'm fairly convinced I'd used Floodgate on all forms of Yubel many times over the past few weeks.
Today I beat some whale who has 3 of this cancer, with him having successfully having used all 3 on me during the duel. That felt great. An UR in a large box that's like the second worst of the game, and there are people who have 3... what the hell is wrong with the world. Whoever I see using more than 1 of this, be ready to have one hell of a tilt duel as I will pull the most cancer tactics possible.
The obvious answer would be counter traps.
Or construct your deck in a way that losing one monster doesn't hurt you too much. Simply tribute it for either a higher level monster or with Enemy Controller etc. There's no definitive method other than having counter traps ready, you just gotta use your creativity.