Open your browser and delete history, cookies, clear the cache.
Open settings-accounts-google-ads and opt out.
You might also carefully examine each of your apps - read the comments in the Play store to see if any are associated with excessive advertising. Try deleting your apps, especially free ones, and add them back one at a time to see if you can pinpoint a culprit.
If all else fails, try factory reset, but I would not go there yet.