Stolen from the Electronic Frontier Foundation:

On Android 

With the release of Android 12, Google began allowing users to delete their ad ID permanently. On devices that have this feature enabled, you can open the Settings app and navigate to Privacy > Ads. Tap “Delete advertising ID,” then tap it again on the next page to confirm. This will prevent any app on your phone from accessing it in the future.

On iOS 

To see which apps you have previously granted access to, go to Settings > Privacy > Tracking. You can set the “Allow apps to Request to Track” switch to the “off” position (the slider is to the left and the background is gray). This will prevent apps from asking to track in the future. If you have granted apps permission to track you in the past, this will prompt you to ask those apps to stop tracking as well. You also have the option to grant or revoke tracking access on a per-app basis.