If your friend has established residency, you will have to go through a legal eviction process.
If no residency has been established, call the police and have the trespasser removed.
The Fifth Amendment to the United States Constitution prohibits the government from making you answer any questions that may incriminate you.
Go watch this video. http://www.youtube.com/watch?v=6wXkI4t7nuc
Refusing to answer questions is NOT obstruction. Lying to the police is obstruction. If you say "I don't know" when you really do, that's obstruction. I suggest using the phrase "I'm sorry, but I don't believe it is in my best interest to answer your questions".