Sorry, just realised that you said you`ve checked the float levels.
As long as you are sure they are right then you`ll have to check the valves actually close. You can do this by blowing down the fuel feed pipe and holding the float valves closed by hand.
Sorry if I`m stating the obvious but, you do know when checking the float heights you start by holding the carbs upright, then slowly rotate them until the valve closes and the float tab just lightly sits against the sping loaded plunger on the valve needle? Then measure from the gasket face to the bottom of the float.

Another thing you can try before pulling the thing apart is take the bike for a good run to clear out any crap in the cases and exhaust. Before puting the bike in the garage, turn off the fuel tap and run the engine until it dies from emptying the float bowls. If it still floods after this it`s a good indicator of the fuel tap leaking.