I don't think this has been suggested yet, but how about you turn the tables on him. I'm not advocating letting him continue to pilfer everything in sight, but how about the next time he's assaulting a Lord from the shadows (perceived or real shadows), the Lord has a lvl 30 epic rogue on his payroll and that epic rogue pilfers everything the thieveing player owns.
Your epic lvl rogue makes a stealth roll 50 and a thievery check 50...the PC discovers that he has not only lost all of his gold, potions and supplies (including food and adventuring gear), but his +2 dagger and and +2 short bow are completely gone and have been replaced with a stick hastily fastened together to resemble a dagger and a curved stick with a string tied to it used to resemble a short bow.
You know, something along those lines where he starts to feel the sting of what he's doing being done back to him. Tell him that he can have his stuff back after he successful completes counseling at the local thieves guild...ok, that last part is just a little joke, but trying to do to him what he does to others may help him see how annoying it is.
GL in resolving this issue at your table.
I see the intent of what you are trying here and while it might be satisfying for the DM I don't think it is the best course of action.
What you are suggesting is actually reinforcing the players behaviour by using the same tactics that he is, only slightly worse because you are abusing the system you are responsible for arbitrating to do it.
A character is far more justified in stealing anything he wants in a world that steals anything it wants from him. Also if you steal from the PC he has a legitimate reason to rally the rest of the group behind him as the aggreived party seeking "justice" or "revenge".
In all fairness this approach stands a good chance of making teh situation worse (in and out of game) not better.