I'm no lawyer, so take what I say with a grain of salt. From what I understand, you cannot be evicted without a court order. Now, just because they don't have one doesn't mean they won't try to throw you out. Some friends of mine had been squatting a house and the police came with the owner and were clearly on the owners side, threatening to arrest them if they didn't leave on the spot.
Changing the locks will make a big difference if you want to fight it. Don't open the door and tell them to get a court order and don't say anything else. It's technically a civil matter at that point and you'll have to go through the court system. That's the ideal scenario, you might be able to stretch it out for a few weeks or months but the end result will be your eventual eviction, but it buys you time and it's extremely unlikely you will go to jail or have any fines.
That is a lot of stress and hassle though, so you might want to just pack your things and move on to another abando somewhere and repeat your current process. It's up to you, just keep in mind that unlike countries that have laws defending squatters, the usa only has tenants rights and the police or even landlords sometimes can just complete ignore those laws with virtually no repercussions and kick in the door and drag you away kicking and screaming. So again it's up to you how much risk you want to take.
@Older Than Dirt has some lawyer experience so maybe they have something to say about it.