or pull the tank off the bike, hold the vent hose up so it and the petcock are the highest point, block it up on the floor like that.
but to answer your question, no it won't ruin your engine. The off valve is for transporting the bike so the carbs don't overflow. even if they do it will still just run on the floor. assuming your seats are good you will be fine.
But really, grab a can and drain it. You should have a Banshee only gas can anyway.