Limit pickpocketing in general through multiple methods IMO. Generally speaking, it's not a very compelling action to take in D&D, and not being able to steal a person's entire inventory would be a start. Limit it to things that you could reasonably take and limit it by the reactions of your companion characters, maybe some don't care but honestly who wants to be grouping with an outright thief? Obviously there are some exceptions in terms of particular quests or characters but in general when you're playing anything but an evil campaign in D&D (which are fraught with problems themselves) your PCs don't like it when other PCs steal.
