Sorry to hear that Brigsy sounds bad. Weather permitting shut the hive that is doing the robbing up for 24 hrs and then open the entrance fully so they are busy defending their own. Put vick above the hive entrance of the hive being robbed and reduce entrance to one bee space. I did not cover entrance with the sheet when I did it just let it hang down to above it.
I think the bed sheet method comes from Beekeeping for Dummies - I'm not convinced it's a good method. They don't give a lot of detail in the book but not giving the bees a means of getting in or out of the hive in hot weather is a recipe for a disaster. The do say to keep it wet (and this will cool the hive) but trapping bees in at any time has its risks.
The best method of stopping robbing is to take steps to stop it happening in the first place.
Strong queenright colonies will rarely suffer from robbing - if you have small colonies keep them in small hive (or a reduced larger hive) robbing doesn't just happen at the entrance - it happens on the frames and a colony spread thinly over the frames will have more difficulty defending the stores than one that is covering the frames quite densely.
Don't leave frames with honey in them outside the hive when inspecting, keep supers covered when you are moving them to inspect. Don't spill honey or syrup. If you are doing manipulations that involve supers or feeding do it at dusk, not when there are lots of bees flying.
The obvious things about keeping entrances small and putting obstructions in front of them have all been stated on many occasions but, if robbing has started, then the best thing to do is move the hive/colony that is being robbed. Ideally 3 miles away but, if you have to, just move it within the apiary to another position and with the entrance facing away from it's original direction. Do this at night so the bees have to re-orientate when the come out in the morning - if necessary and weather permitting seal them in for a few hours to stop them returning to the old position immediately. Then, put an empty hive containing a frame with a small amount of stores in place of it - at a pinch a cardboard box with a small hole in it for an entrance with a saucer of honey in it will do the trick - the bees doing the robbing will stop when they have emptied the stores from the 'hive' being robbed. Once it is robbed out take it away and if you have one move a strong colony into the position.
It's not ideal, moving the colony within the apiary, but it's the best of a bad situation.
The key is very much to act quickly, bees are great communicators and opportunists and a day is almost too long .. a strong robbing colony can empty and decimate a weak hive in no time once they get to grips on it.
