Thanks to all of you for commenting.
During the shoot I was on the fence about the graffiti, thinking about the square crop solution just like some of you did. But ultimately I decided against it to keep the image authentic looking. Graffiti on decaying industrial buildings have become so common that an image of such property without them loses credibility.
However, the rainwater pipe between the window simply doesn't belong. Not only does it attract attention, it divides the image and thus brings more attention to the graffiti than it should get. So I simply removed that thing, straightened the image and cropped the right side off a bit to move the window out of the smack center position. Now the tree/window combo gets the attention it deserves and the graffiti is no longer a distraction.