According to the artist, Andrew Morrison, he knows who did it because "he was stupid enough to sign his name."
The words DAPKILO were painted over, said Morrison, who sent me the photo you see above. DAP is the name of a crew of graffiti artists. It stands for Down Around Pike, said Morrison, and KILO is the name of one of their artists.
It remains to be seen how this can be mitigated but these were not the actions of "graffiti artists" but of vandals.