You can remove them for free by restoring your purchase.
Tap "Restore Removed Ads" in the settings screen. (If you have a "Premium Service" subscription, tap "Restore Subscription".)
In rare cases, this may not work. If so, please try the purchase process again.
You will not be charged twice as long as you use the same Apple Account, so please don't worry.
If it still doesn't work, please contact us with the details via the "Contact Us" button at the top right.