Graffiti is an art form as old as humanity itself. People have been leaving their marks on dwellings since the Chauvet Cave paintings 44,000 year ago. When done thoughtfully and with an owner's consent, graffiti can serve as a snapshot of the culture. The problem with most graffiti artists, is that unlike other painters, they don't own the canvas they are painting upon.
In this case, consent is what separates a mural from a misdemeanor. It's one thing for a building owner to commission a graffiti art piece on the east side of their building. It's entirely another for someone to buy can of Krylon and write their Twitter handle on a building in the middle of the night. And if Banksy himself tagged my house without my consent, I'm pretty sure I'd be miffed.
Cleaning graffiti off of a building is an incredibly difficult task, especially on masonry. The combination of porous, absorbent masonry and the high solvent (VOC) composition of spray paint means that the graffiti will likely soak deep into the brick or block, making normal power-washing a fruitless endeavor. You can try chemical stripping, which can take off the paint from the surface, but residual paint can remain in the pores resulting in graffiti "shadowing." You can also sand-blast the material off, but you now risk permanently damaging the substrate or worse, etching the graffiti into the masonry itself.
So what can architects do to make sure that their building is saying what they intended? An ounce of prevention is worth a pound of cure in this case. On masonry buildings, using an anti-graffiti coating like Prosoco's BlokGuard and Graffiti Control II or SC-1 (Sacrificial Coating) will ensure that even if your building does get selected to be an "art piece", that clean up will be a breeze. These breathable materials, create a thin film on the surface of the masonry which blocks spray paint, grease pencils, etc from soaking into the pores. Also, you don't have to treat the entire building, the vast majority of graffiti happens within 8 feet of grade. So unless Andre the Giant is vandalizing your building, then you don't need to treat any higher than that.
If you apply these coatings correctly and your building still gets tagged, simply clean off with Prosoco's Graffiti Remover.
In the end, taking small protective measures will allow your building to speak for itself.