Hi there,
I'm afraid that the Mobile Ads SDK does not have any API methods to automatically close a Rewarded Video ad upon finishing. While I am not able to comment on other Ad Networks, Rewarded Video Ads from AdMob do show an alert dialog when a user tries to close the ad when the ad has not yet finished, as shown on the attached screenshot. However, this is not customizeable as well.
Let me know if you have any further questions.
Regards,
Joshua Lagonera
Mobile Ads SDK Team