> a very hard problem to solve

Is it? Honest chargers who go out to the boonies to recover truly hard-to-recover scooters will pick up scooters which turn red from all over the place. Dishonest chargers who kidnap scooters will have scooters who all turned red suspiciously in one place (the charger's home).

Developing an algorithm to find the dishonest players should be relatively straightforward, and after dishonest players are found, they should be banned.

Yes, this seems largely trivial for a moderate sized company.

Will it be perfect? No. Will it be effective? Yes. Will it make a great HN blog post? Absolutely

  Dishonest chargers who kidnap scooters will have scooters
  who all turned red suspiciously in one place (the
  charger's home).

...until the dishonest chargers figure out they can easily block the GPS signal with a few cents of tinfoil when they grab the scooter off the street.

You can catch that too.

How will you robustly tell apart vandals from dishonest chargers, when the latter are incentivised to emulate the former?

Well you just ban them. You might catch some innocents but that's their right. If someone has a habit of turning in a whole bunch of scooters that didn't have GPS available for X days before, ban them. A legitimate charger should have a certain percentage of scooters where the GPS was available up until charging (1-2 day turn around I'm guessing, or there's some fundamental flaw with the charging system), so I'm sure there's an algorithm that can remove a majority of the abusers with minimal casualties.

You can never solve gaming the system entirely, the goal is just to make it harder than it's worth, and hopefully the abusers move to some other scheme.

  If someone has a habit of turning in a whole
  bunch of scooters that didn't have GPS
  available for X days before, ban them.

You've just banned your best guy, Wader Wayne, the only guy in the city who was retrieving scooters that vandals had chucked in the river.

Now he's complaining on your employees' subreddit everyone said he was doing a great job and his numbers were really good, that no-one else should invest in equipment to do the same work, and that you've ruined Christmas for his family by cutting off their only source of income. The Guardian is going to interview him for their article on job insecurity in the gig economy.

All of these parameters are tunable. You don't have to get rid of 90% of the dishonest catchers, if even 50-70% will make a big difference. Your argument would have more merit if the majority of unrecoverable scooters were due to vandalism, but the facts bear out that the vast majority are due to hoarding.

From the GPS signal being blocked some time before they charge the scooters.

You think vandals never chuck scooters places they can't get GPS signals, like in rivers or metal dumpsters?

You use statistics.

These hoarders display obviously anomalous behavior. Overtime, Bird will be able to weed out this fraud.

The only option I see is banning somehow by law/agreement, and taking legal actions to people who 'steal' them.

There's something really funny about the libertarian mindset here:

  - Screw regulations! Disrupt! Use public property for private gain, but then
  - Let the hammer of the law crash down and the state deal with those scundrels if there are unintended consequences

Theres' a smidgeon of hypocrisy here, don't you think?

The libertarian mindset is pretty clear: There should be a small number of regulations, with a specific scope (e.g. protecting against physical harm and protecting property), which are clearly enumerated so that anyone, across most levels of education, can easily and reasonably determine if the regulation is fair or not.