If they have a warrant, they can force you under threat of legal action if you don't comply. If they don't have a warrant, you can claim the 4th. If they try to get you to divulge the password/code/secret, you plead the 5th. If you let them in, well... Politely tell them they are no longer welcome. Please leave. If they don't comply, they are trespassing (unless they have a warrant, in which case none of the above applies and you're probably going to jail, wear clean underwear).