Personally, my first attempt would be to ally with the theiving player. They're obviously willing to risk their tail for gain against other players. That kind of daring could make them a useful companion some day. Example: Months later you get in a conflict with someone else. You message the player who theived you: "Hey, there's this guy hassling me, and I just so happened to have cleared out all his runes. I know you like thieving..." and, well you get the idea.
If that doesn't work I would turn to my alliance or, only if I was unallied, the GC, to help me negotiate a fair repayment based on how hard and often they hit me, how experienced they are at the game, etc.
Personally I think the important thing with all hostile actions AND their rebuttals is purpose. At one extreme, we have the PvP-hungry whiners who desire open PvP along the lines of Log In>click World Map> think "Hm, what random player below my population shall I attack for fun today?" At the other we have "White Knights" in Illy who jump into incidents that they have no business in and even less to gain from as the initial offender had in attacking.
On both sides of these conflicts, we have players engaging in military and diplomatic actions that offer them no significant gain other than to satisfy an urge, just because they can. The PvP whiners fight to satisfy their need for conflict and competition, the White Knights their desire for peace and heroism. Does it really make one side better than the other just because one is fighting for "justice" and another for "action"?
Fortunately, I think most Illy players do not fall on either extreme but occupy moderate areas of the spectrum, and most actions are fueled by purpose.