I'm afraid this forum can only assist you with technical concerns specific to the code implementation of the Mobile Ads SDK features.
That said, you may refer to
this section on how you can clean up banner ads via the Destroy() method. Also, you may refer to
this code snippet on how it was implemented in one of our sample apps. On the other hand, for the code implementation of doing any action (e.g. cleaning up views) when changing a scene in Unity, it's best that you raise this to the experts in the Unity developer forums as they can better assist you on any concerns on general code implementation in Unity.