I'm not sure how well those citronella collars work. I've heard they are fairly effective with minor issues, such as excessive barking. But I think your pup needs to learn boundries; with correction, not out of fear of being sprayed or zapped. I have known people who have used the "training collar," but only under the guidance of a trainer/behaviorist familiar with their use. Used incorrectly, you can create all kinds of other problems. Is this type of behavior new to your dog? How long have you had him/her? I think consulting a trainer first would be the best idea. Have you ever attended obedience classes? Left uncorrected, this type of protective/territorial behavior might escalate, and you don't want to wait until something really serious happens; someone gets injured. I would at least start with introducing the dog, on lead, to any new visitor/stranger outside of the home itself; say, in the yard or driveway. Keep treats in your pocket and give the visitor one to offer your dog while you have him on lead. Keep him on lead in the house when new people arrive and keep him by your side. Tell him to sit and have the visitor offer him a treat. At the first sign of any aggressive action, give him a strong correction correction jerk with the lead, "No!!!." Cattle Dogs are known to be spirited and independent minded, but you still need to establish that while it is his home, you are alpha and in control. Hope this helps a little. As I said, I definitely would consult a trainer. I hope you and your pup can work this out