well first of all try to see if they are actualy barking while you are away...and try to get the cause of it too.. so if you have a voice recorder on yoru computer or hand heald that would work great.. you need two of them.. put one inside anywhere .. and the other outside.. the outside one will record other dogs and people outside your apartment knocking whatever to prevoke your dog to bark.. and the inside one will see when your dog barks.. the recorders if you get a decent one should record for 2/4hours longer the better..

that will only help you understand your dog and why the barking is happening cus if someone is knocking on your door and thats only when he barks well tell your nabbers to stop knocking on the door and have them listen to the tapes.. and then tell them to stick it your dog is doing its job

if that is not the case .. then you can try a bark caller the one I suggest is the one that either sprays citronella out when they bark or one that makes a high pitched noise when they bark.. both are humane.. seeing you have several barkers around you I would suggest using the citronella one cus the high pitched noise one may be set off by the other dogs barking and thus be punishing your dog when he is not barking.