Rule #1 is that if you don't want anyone to know your business don't even tell one person your business. I understand that if I tell someone at work something, even if I presume it's in confidence, soon everyone will know.
Rule #2, you can't change anyone but yourself.
Also, you might not want to sweat the small stuff. We spend a lot of time with our coworkers. While asking a ton of personal questions isn't appropriate, is it really a big deal people know you have a new car? Getting a new car is a big event for some of us and when I got a new car, I was happy to share my good news, as well as when I travel or do other things. I certainly don't give away intimate details of my private life, but like I said if I do, I understand everyone will hear about it. But I do have to allow you your preferences. Some people aren't comfortable with small talk.
Bottom line is to set boundaries and ignore people, or just be honest and say "that's nothing you need to know". Eventually people will get the clue and leave you alone.