When someone arrives in your home uninvited, it is often upsetting. Many times, it is an innocent mistake. Your home might have been the landmark that was a previous landmark of something else. Land owners change all the time and it is useful to check to see if your landmarks are still accurate.
First, let the visitor know that it is a private home and ask the person or people to leave. Most of the time, considerate people will apologize and leave and you will never see them again.
Secondly, you may want to check your access for your land parcel. If it is a LINDEN home, you can lock and restrict access through your home controls and in your ACCESS tab. You can restrit access to only your friends.
Third, if you want ultimate security, you can invest in a land orb or radar tool that restricts unwanted individuals from your land. You need to be careful of this because when you invite new friends over, you will have to ADD them to the access list and take their name off etc. Some houses also have these security tools built into the homes as well so you can look for this feature when you are house shopping.
You also want to be wary that people can often access your house through camming and clicking on menu driven objects. So if you have pose balls around, they can cam through the window, click on a pose ball and gain entry. If you do not have an orb that ejects them right away, they can end up squatting in your house.
Be wary also of leaving your build rights open too. You can build in various levels of security for your home, but you need to realize that second life is a culture of exploring so now and then, expect a few visitors.
take care!