If the mayor was serious about catching Banksy then he would go to the phone companies and get all of the phones that were within a 200 metre radius on the night that the artwork went up for each and every piece. Then he could put them in a database, get his 'SQL for Dummies' book out and select just the phone records that are common to all incidents.
He can then go back to the phone companies and get the billing details for Banksy and his entourage. He could also ask them to let him know exactly where they are, follow them and catch them red handed doing their next piece.
Personally I see Banksy as a cartoonist rather than as a graffiti artist. He does not have a formal arrangement with the papers to syndicate his work, he does not even have to churn something out every day. Instead he gets his work prominently shown in all of the British papers, reaching an audience that no other cartoonist can. He has Robin Hood grade street cred. due to this audience reach.
Whereas other cartoonists use pen and paper, Banksy uses the side of some house or another wall as the medium. It is an intermediate form much like the conventional cartoonist's paper is. Although of value to the crazies that go mad for such things it is of no value to Banksy if his aim is to get his work into the paper, to reach a mass audience.
As for the people who have inherited the work, they could just let the boring 'tag' graffiti artists vandalise the Banksy masterpiece as quickly as possible, whilst there is still the media interest. It will then be known that it has been destroyed and the troublesome visitors will cease to turn up. They can then paint it over, to restore their property back to normal and get on with life.
Getting back to the mayor, if the trick works for catching Banksy then it will probably work for all of the inane tagging losers out there. So long as citizens report new tag-vandalisms in a timely fashion then the police should be able to get a reasonable sized list of phone numbers to work with.
I have known and known of 'tag' graffiti artists in my time. I still don't see why they are so determined to do what they do and for so late into adult life. I feel sorry for them not having any meaning to their 'art'. The strangest thing to me are the 'tags' put up in some foreign town. Imagine going to another country, a place you do not live, just to paint your tag up on some walls that you are not going to see again.
With a Banksy it cannot be said to truly exist until the papers report the new birth. With small time tedious tagging types they get Facebook instead of the media to show off their efforts. Invariably those that tag post their tags online somewhere. The phones can lead the authorities to where this is and get the evidence needed for prosecution.
