Hello:
Your dog has Canine Possession Aggression. Better known as object guarding. This is VERY COMMON!!!
Begin by taking away the trigger points. In your case, when people come over, ball gets put away and doesn't come out till they leave.
You would do much better introducing other dogs outdoors on "Neutral grounds" as well. Then when they get acclimated, the other dog can be allowed to come into your office. Supervise the dogs and watch their body language. Allow them to be together for very short sessions and end the sessions on a positive note for both dogs.
Next, you need to begin to practice NOTHING IN LIFE IS FREE! Your dog is running the show.
Here are some exercises that will help with canine posession aggression. My own dog has it and I have done this with her. It worked great. The dog needs to learn to listen to your commands. You taking charge and working with your dog as well as doing basic obedience will help you and your dog establish a stronger relationship with you being in the drivers seat!!!

Some dogs learn to protect certain objects usually as a reaction from their siblings taking and tugging objects away at an early age, or we teach them this behavior by our own actions and reactions.

How do we work with this?

It is important for your dog to view you as the provider of all good things. Begin to practice Nothing in life is Free. (you can google search this) You need to take charge and ALWAYS control the dog's environment. You run the show, not the dog! Keep all but one or two toys up off the floor and take the others down only when you want to play. Make sure you offer an item with a command like "Take it." When you are tired of the game (you that is, not the dog), tell the dog to "Drop it” Give him another item or treat in exchange, and then pick up the first object, putting it away.

Make a list of all the things your dog really enjoys including food, toys, treats and activities, rank them in a hierarchical order, In exchange for dropping the first item give your dog a second, "better" item. For instance, if tennis ball retrieving is third on your dog's list, reward him with cheese, cut up hot dog or piece of chicken for dropping the ball.

Teach a Cue Word

Teach the words “drop it” do it in a fun way with a happy high pitched voice. Start by allowing him to have something that is not so valuable then trade with him for his best treat.

Take away all objects the dog is guarding, that could be toys, socks chews, bones, pigs ears or sleeping places - . Do not allow access to these precious resources a number of days.


Set scheduled times to repeat the "Drop it" exercise at least four times a day but also just do it in opportune moments. Gradually up the anti of treasured goods. Over a period of time the dog will start to look forward to your approach and game. It is at this time that you give your dog the object and walk away, at first come back immediately and trade gradually making the time and distance you walk away longer, until you clearly see the dog is having no problems with your approach at all. . Then only give a treat every third time, then every tenth etc… , , take the object away and immediately give it back extending the period on this until the guarding behavior is diminished.

Hope this info helps.

Susan