"My response this time is to collaborate with my tagger, and not return the piece to its original state, but see what happens," said Stromberg, 50, while laboring to repair his mural.

"The tags inspire new visual vocabularies I would not have thought to paint, had the piece not been tagged."

Tagger response: "Its war"

In August, "Blake" returned with a vengeance to Clarion Alley and ominously wrote in black letters across the repaired mural: "THIS ISNT ART. ITS WAR. BLAKE."

"I wish you could get ahold of Blake, my tagger-collaborator, but I've no idea who, where, what, he is," Stromberg lamented after the fresh assault.

Stromberg's Facebook friends analyzed the tagger's motive.

"You could think of it in terms of mental illness. It's Tourette's -- but they have to write their name over and over maybe," posted one colleague.

"He is expressing some sort of superiority -- read: 'internalized inferiority' -- over a real artist. I was just marveling at the tags on the toilet seat at a local establishment. How low do you have to be, to want to tag that?" someone else wrote.

"I really just don't get why he has chosen to target Mats Stromberg of all people. A legendary, grimy, old school Mission dude, underground pioneer," another dismayed artist wrote.

"This same stuff has happened to mine as well. It's a bummer, but it is good to keep in mind that anything we put out there into the public is ephemeral, meaning it could last a long time or no time," that artist said.

"Stick a nanny cam on there, get a mug shot, and then paint the dude's face on there, wearing a T-shirt that says 'I'm with stupid' and an arrow pointing the guy's head," suggested another post.

Attack on establishment art

"I'm guessing they see a legitimate commissioned or permitted works of art, as being part of the establishment," said Stromberg.

He was referring to the alley's envious status of enshrining some of San Francisco's best art, organized by the volunteer artists' collective Clarion Alley Mural Project (CAMP).

"Sadly, CAMP has helped to contribute to the extreme gentrification of the Mission District over these past two decades," wrote artist Megan Wilson, a CAMP "core organizer" who defends the alley's roots.

"What started as a neighborhood-based project committed to diversity and inclusion, is now a magnet for lots of folks hoping to profit off of the image that CAMP has created -- from the developers and real estate agents who use CAMP as a selling point for the 'cool, hip Mission experience,' to those who use the space for fashion shoots, to corporations hoping to include the 'gritty urban street art' image to sell their products, to any number of paid tours by folks unrelated to CAMP, spreading misinformation about the project, artists and murals," she wrote on her website.

No control over taggers

Many of San Francisco's large murals are the result of serious negotiations between artists and wall owners -- including shopkeepers, residents or city officials -- to ensure the painting remains visible and not erased for violating private property.

But no one can control malicious taggers.

"I believe taggers will always state their voice," said Perez-Boza. "Whether it's tagging a white wall, a mural, local mom and pop, venues, billboards, public transit or corporate buildings."

People attack murals because "they knew the artist, and maybe that artist they knew 'sold out.'

"Or they at one point were in the same crew, and now they are in different crews.

"Or jealousy and envy. Or they just don't care.

"Or they can be sending gang messages.

"I don't know what the punishment should be, if someone intentionally spray-painted over a mural.