You need to make any purchase about once a month to get rid of ads.

It’s important to note that only forced ads are removed. Ads that play in exchange for rewards are a different type of ad and will remain available.
If you made a purchase on one device but are playing on another, ads will not be disabled.

If ads don’t turn off after a purchase, please try the following:
1. Update the game to the latest version.
2. Restart the game.