The Invasion: You catch a dinner guest poking through your bedroom or perusing your medicine cabinet.

The Defense: Even the nosiest person will be embarrassed to be caught in the act, says etiquette writer Ceri Marsh. You can let the person save face by saying, “I’m sorry. You must need something. Can I help you?” Then the guest has an easy out — he can respond that he was looking for an aspirin or some other common item. As he or she follows you to find the item, you might gently close the door behind you.

The Invasion: A neighbor regularly shows up on your doorstep for coffee, unannounced.

The Defense: Play it straight. “Say, ‘I’m sorry — this isn’t a good time, but thanks for thinking of me,’ without inviting the neighbor into the house,” says Jane Adams, Ph.D., a psychologist and the author of Boundary Issues (Wiley, $25, www.amazon.com). You could also plainly admit that you prefer scheduled coffee dates to impromptu visits. If you work from home, you have a built-in excuse for turning away any company: “Listen, Mary, I have this deadline, and I have to work on it. Maybe we can get together later next week.” Your neighbor doesn’t need to know what is occupying your time (if you’re simply relaxing on the couch, then so be it). She only needs to know that you are not available.

The Invasion: The cleaner, the dog walker, or the sitter moves things around in areas of the house he or she has no business being in.

The Defense: “Absolutely address the situation,” says Debra Johnson, the training manager for Merry Maids, a national home-cleaning service. After all, you’re paying for the job, and communication is the key to getting what you want. Guy Maddalone, the CEO and founder of GTM Household Employment Experts and the author of How to Hire and Retain Your Household Help (gtm. com, $20, www.amazon.com), says, “Whether it’s a nanny, a dog walker, or a house cleaner, that person wants to be successful in their role, so you need to explain the policies in the beginning to set them up for success.” Schedule an orientation meeting with the employee at the start and explain your rules, including the places and things that are off-limits. You might even take this a step further by creating your own employee handbook. That way, you’ll both have a clear, tangible reference to consult in the future.