I have had to encounter this. I have two suggestions.
1. EXERCISE. My Cavalier gets super annoying and will stand facing his human family, barking at us. He also used to bark any time anyone was within proximity of our apartment, indoors or out, so basically ALL the time. I started waking up an hour earlier and jogging with him (OK more of a really fast walk) for 45 min to an hour each morning and same thing at night. If you can't do this, hire a dog walker or neighborhood kid to do it. Tired dogs don't bark! Its actually funny, because he'll let out little barks and half growls now, but he is all stretched out sleeping so he is too tired to give it the full volume concert he used to do.
2.
Busy Buddy - Twist n Treat Dog Toy - Medium | Treat Dispensers | Toys | Dog - ThatPetPlace.com
Specifically for seperation anxiety barking - I fill this toy with treats before I leave and the dogs 'work' to get the treats out. I just use some of their regular 'kibble' portion of food so we don't get overweight.
Good luck!