Ads in MIUI can be stopped from appearing on your phone screen by maintaing following steps...
in MIUI Browser:
settings-Advanced-Top Site orders-Receive Recommendation( Turn off this )
settings-privacy-Recomendation for you(turn this off also)
in Theme App :
Settings-Show Advertisement(turn off this)
In Security App:
settings-Receive Recommendation (Turn Off this)
in MiUI Music app:
settings-advance settings-Receive recommendation (turn off this)
in MIUI Video app:
settings-Online Recommendation (turn off)
in Miui File manager:
settings-about-Recommendation (turn this off)
and from settings
Additional settings-Authorization and Revocation- msa(turn this off)*
*this settings can't be turned off if you are not signed in your MIUI account in your device and if the device is not registered with your ID.
**all of the above mentioned steps are personally tested on Miui 9(Mi max, Redmi 5a and Redmi note 5 pro) and Miui 10(Note 5 pro). **
thanks for reading