The advertisements that appear in landscape mode do not provide an "x" to close them at the end of the game forcing players to close the game entirely and forfeit the reward (topaz or crystals) they should receive for watching the advertisements.
Technically it will appear a 'X' when finish playing ads.
Could you tell us your phone model, Android or iOS?
I have an iPhone 6. The "x" does not appear for advertisements that run in landscape mode (top of the advertisement is on my left see pictures). The only way to return to the game is to restart the game. Consequently I don't receive the topaz despite having watched the advertisement.
Do you see an "x" on this screenshot?