Beside the point. You wouldn't take it upon yourself to change people's clothes on the street, no matter how much better your fashion sense is.

It's more like intercepting spam and replacing it with a haiku

The difference is, your inbox is yours. Those walls belong to someone, and if they want to turn a profit renting the space to advertisers, that's their business and no-one else's.

edit: Questions of legality aside, I mean.

Technically, my inbox is owned by Google. I just look at it a lot. Kinda like billboards.

Again, the original postings weren't allowed (like spam to my inbox).

There are some who argue that billboards are like noise pollution, but with your visual instead of auditory space. I think that is partially correct, and so I don't mind this kind of corrective action.

Note that this kind of of public art display is exactly what makes NYC a vibrant place to live and other areas really boring. That kind of vibrancy definitely has economic value.

"NPA Outdoor operates over 500 street level billboards in NYC ranging in size from approximately 4’x4’ to 50’x12’ all of which are said to be illegal."

If you are making money by breaking the law and annoying people then I have far less sympathy for them than if their legal private property where damaged. IMO, It falls under the category of destroying illegal campaign advertising on public land.

Are you a cop or a judge?

No, but I might end up on the jury of your peers.