ADBannerView sometimes doesn't disappear

I use ADBannerView to display iAd banners on the bottom of my Unity3d for iOS game. When changing the scene, I want to remove the banner. This is the code I use to remove the banner:

             if(_bannerIAD!=null)
             {
                 _bannerIAD.visible = false;
                 _bannerIAD.position = new Vector2(0,Screen.height);//just to be sure
             }
             _bannerIAD = null;

so, I'd expect that _bannerIAD.visible=false should be enough. To be sure, I also try to position it off screen. But still, the bannerad sometimes sticks around and is still visible in the next scene. This doesn't always happen but only sometimes. If the banner ad does not disappear, it will not be clickable subsequently, but it's still visible.

Does anyone know this problem and knows how to work around it? How can I destroy the banner for good?