Just got this one when entering the forum. It was quite frustrating, no “x” to click, and it tries to force you to sit there for 30 seconds. If you touch the screen, the timer resets.
FYI, just hit the back button.
I’m all for sitting still (meditation), but trying to force someone to do nothing for 30 seconds with an advertisement does the exact opposite lol.
You can make a small one off payment and that removes all ads.
Sounds like you have it sorted though if hitting the back button stops it playing the full ad 
Although I really rate the Calm app, learning to sit and be patient is a very useful tool in my sober toolbox!
It’s just an add.
Pay the little money or, like I do, just ignore it.
Or perhaps install it and chill 