Basically, the EULA should (and likely does) have some terms against that. That is to say, scooters that are fobbed out by their user should be left in such a way that their are accessible to other users. Identify the violators and charge them fines and/or terminate their accounts.

Keeping scooters in homes and garages can be allowed, provided the scooters are "on the clock": in the middle of a booking that is being paid for.