Using ubertubes to block opponents from scoring

I know you cannot enter the opponent's scoring area, but does anyone know if it's legal to throw an unused ubertube into the opponent's scoring area, so as that it blocks the opponent's robot from moving fully forward, preventing them from scoring? If done correctly, it could stop a lot of elevator mechanisms from getting close enough to the pegs to score. Though they can be pushed out or picked up and moved, it would take time away from your opponents and find a use for the unused ubertubes!