I would start by building a super solid "go to your bed/mat command". The dog learns to go to its bed and hold a stay there until released. Teach by luring on to the bed from only a few feet away and only asking the dog to stay for a few seconds. Gradually build up to holding a long stay and to going to the bed from across the room. It is important that being on the bed (or you could also use an open crate) is a good thing so reward well for that behaviour.
Then after you have that you can then say "thanks for the warning", take a quick look to see what they are barking at and then ask them to go to the bed. It will help break up the obsessive part of the barking and eventually could lead to the dog just doing a short alert bark and then going to the bed by itself.
Goodluck
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