I have an app I've created with AppInventor at beta.Thunkable. I have AdMob set up, and the ad shows up just fine. There is a button where if you press it, it changes the AdMob Banner's "Visible" option to False, but the ad doesn't go away.... it is still there, and I can't figure out why.
Also I have tried to use an Interstitial ad with AdMob, but there is no option to un-Visible that either???
I have run into this issue multiple times. When an app is particularly large, there just seems to be no way to get around it in App Inventor. However, there are two current solutions.
The first is to change the visible option to an enable option. You really don't even want it to be enabled when the add is supposed to go away, because even if it does turn invisible, by some glitch the user could accidentally press on it. Disabling it has always worked for me, and could provide a less buggy solution. Long story short, change the visible block to an enable block and set enable to false when you want the banner to go away.
The second option is to just place a blank canvas or object over it, which you may have already thought of. Pretty self-explanatory.