That's basically the premise, yes.

https://en.wikipedia.org/wiki/Assassination_market

Spraypaint a private key (or any unique symbol, really) to prove you did it.

Would need to be an agreed-upon message encrypted with the vandal's private key.

1. Person wanting defacement would indicate the amount for the bounty, along with the message to be posted on the defaced property

2. Vandal contacts payer and agrees to conditions. He/she also gives the client their public key

3. Vandal performs defacement, then add the specified message to it, encrypted in his or her private key

4. Vandal notifies the person paying the bounty that the job is complete, sending him/her a picture of the defacement with encrypted message

4. Payer decrypts message, verifies it matches, then pays out the agreed upon bounty to the vandal

Cryptotags.

To be honest, that's what spray tags look like to me already, random, meaningless, unique. I've photographed hundreds of tags and have never seen two alike.

Now, hacking the ad screen with a virus that displays a unique artwork... that'd be cool.

I'm hoping the defacement would be more contextual and sardonic, like devil horns on people or changing Coca Cola to Cloaca

Privkey QRs would be a bit ugly

I recently saw a giant plush coca-cola can with the text changed to "cloaca" in some guy's car. Is that a known joke? I thought it was pretty funny