Damnatio Memoriae: Geta
Damnatio memoriae is one of those interesting concept debated by scholars - to what extent did emperors try to remove records of predecessors they disliked? The memory of Agrippina, for example, is meant to have been removed from the record, but archaeologists suspect that this meant her statues were removed from public view and stored in warehouses rather than destroyed as there are almost more examples of them than of any other empress.
Geta is one of the few emperors whom we are sure suffered damnatio memoriae - his name was erased from an inscription found at Matilo (Leiden), his image was erased from a tondo depicting the family of Septimus Severus ... and now another inscription has been found from Alba Iulia in Romania, where again Caracalla erased the name of the brother he'd had murdered (news story). Although the name was crudely scratched out in the Leiden inscription, the photo above of the Romanian inscription shows that it was neatly erased, and so as a result of an official measure rather than a mob reaction.
I do believe that a lot of the destruction of sculptures may well have been by 'mobs' in Antiquity, just as it is today - for example Saddam in Iraq, and more recently statues of Gaddafi in Libya this summer - but when the destruction was on official monuments we can be sure that it was endorsed by the emperor and his regime. Another example of a 'vanished' Geta is this panel from the Arch of the Argentari in the Roman Forum, which today only shows Caracalla making an offering; the 'shaddow' behind him outlines the body of Geta which was cut away.
More about the damnatio memoriae of Geta can be found here (pp. 168 ff).