So you keep tagging the same wall, and, try as you might, your work keeps getting buffed. What do you do? If you happen to be writing on wood, and you happen to have a lot of time on your hands, you might consider the FireWriter by the artist and designer Lucien Langton.
Rigged together using an Arduino, a computer running Processing, a hacked inkjet printer, and a Dremel blow torch, the FireWriter takes black and white digital images as input, then recreates them on the wall in FLAMES. From Creative Applications:
A black & white image first has to be fed into the Processing script, which dynamically writes a Wiring code that can then be compiled and sent onto the Arduino board. The printer then burns dot by dot (pixel per pixel) on the horizontal axis. The machine is equipped with step by step wheels that enable the user to lower the printer one line at a time on the vertical axis on any support. It works on various types of supports such as wood, paper, walls, plastic, fabric and others. The duration, strength and precision of the flame can be controlled in real-time manually with a pitch.
Remember that Google Image toaster from a little while back? This is kind of like that, but much, much larger in scale, and instead of burning pictures onto bread, it burns them onto walls. Just watch the video above, then start imagining the kind of havoc a graffiti writer could wreak with one of these things.