Annoying adverts showing on top of other apps
The first reason for these annoying ads might be because of your browser when you have accidentally clicked on any of adds shown.Those malicious adds may install some apps in your ROM(internal memory) which may not be uninstalled so easily or will re-appear every time you connect to internet.
So, you could take backup and restore the phone to factory settings.
I did the same way when i was attacked by these malicious apps.

PS: these type of apps may import your photographs and other data. Please be cautious and do factory setting ASAP.