Technically this is vandalism of a govt building.

If they're so concerned about cost, why not do what you'd do with any other vandal with a spray paint can...

Immediately paint over it with grey paint to neutralize the "blight", then efficiently do a deeper clean / removal / repair the facade.

Seems like they gave the Banksy very special treatment of fence, guards, etc. and now are paying for the side effects of that.