Why dont you guys keep a pistol with you..once he has your phone and his back toward you. Shoot at the entire group mercilessly..some incidents like these..and these people will learn a lesson...
Another idea I have often though is to have a replica of expensive phones with a C4 cartridge and micro radio transmitter good for about 100 meters. Once he is running away with your phone press the button...few incidents like these and nobody will dare snatching phones again.
The Sialkot brothers type treatment is a good remedy for such thugs..