Fly-posting is the illegal display of advertising material on buildings, highways or highway land (includes street furniture, road signs, railings and grass verges) without the appropriate permissions and consent of the landowner.
You can report a fly-posting problem to Hertfordshire County Council here:
If you wish to place or affix posters on any structures on the highway or highway land, you must do so with the appropriate permissions and consent of the landowner.
You have a duty to check that the appropriate permissions and consent have been granted before attaching any advertisements or signs.
Hertfordshire County Council have responsibility for signs on the highway.