It can sometimes take a few minutes after an app update for the Play store DRM to run and for Play to recognize that the app has been purchased. If you are still seeing ads after a time, the first thing I'd do is reboot your device.
If that doesn't make a difference, I would go in to the application manager/manage applications in your device settings and locate the Play app and then clear the cache for Play.
This should force the Play app to re-query their database for purchases.
The above scenario can also occur when reinstalling the app.