To stop them: When they grab you from behind just press Shift+RMB to break grab & then Ezio will flip him on to the ground & kill him by pressing LMB with Hidden Blade.
To make them Attack: There is no scripted method to make them attack you as they randomly appear in the map. But they normally spawn where you see patrolling guards in large number, like 3 or 4 patrolling guard groups. Just walk around in the streets near them & you'll eventually find one.
Good luck!
Really? I remember being grabbed by a stalker from behind a while back, it was the first time so I was like: HOLY ~censored~ WTH?!
And all I did was press LMB in... 'shock'. And Ezio stuck his hidden blade right into his face. Maybe I got lucky?