I had a weak hive fall victim to multiple robbing episodes this year (my first). I've found that once robbing starts, it is very hard to stop - the robbers seem to remember and constantly return to the easy mark, and the victim hive gets weaker with each robbing episode becoming yet more vulnerable to robbing. This particular hive was hit so badly on multiple occasions, that it now may not survive the winter.
I finally stopped the robbing by building and installing robber screens modeled after those shown on the U.C. Davis entomology website. The U.C. Davis link no longer works for me and I can't post a URL since I am new, but photos of the screens can be found at: curbstonevalley.com/blog/?p=7329 (just google it).
The design of the screen does little to restrict home bee traffic (unlike some commercially available robbing screens), but the robbers have been thwarted by it for the last two months. Since the screen allows free flow of the home bees, I see no problem with leaving it on the weak hive until they finally completely die or build back to a strength sufficient to protect themselves. In fact, I am thinking that I might just install such robbing screens on all new hives until they build up to at least one full deep.
Good luck and I hope you save your hives!