Alta Planning + Design has an interesting post on dockless bike share: The Dockless Bike Share Revolution; Is Dockless Bike Share Right for Your Community?
The Social Bicycles Tower Bridge Preview is a dockless system. Designated hubs are created both by geofencing (setting up a boundary for hubs and for the system) and designated bike racks, with a $2 penalty for leaving a bike outside a hub, and $20 penalty for leaving a bike outside the system boundary. But it also encourages return of bikes to hubs with a $1.50 credit. These and other issues are discussed in the post, worth a read.