You can only use force if it’s immediate, necessary, and proportionate to the threat. Pickpocketing is non-violent, so stabbing or seriously injuring someone would almost certainly result in you being charged with assault or worse.

Acceptable actions: Yell for help, push them away, grab your belongings back, or restrain them until police arrive—but only if it doesn't cause serious injury.

Bottom Line:

USA gives you more leeway in responding with force, but you still can’t legally maim or kill someone for stealing your wallet unless your life is in danger. France and Italy are stricter, and using even modest force for a non-violent theft can land you in jail.

(http://www.autoadmit.com/thread.php?thread_id=5730758&forum_id=2)#48968575)