Very simple. Just evict. Base it off of unauthorized tenants moving in. No judge is going to buy their excuse of permanent campers in the backyard not being unauthorized tenants and let them continue based on a technicality of "We're not 'living' in the house!!! We can stay here as long as we want!!!!". If it's been going on longer than 2 weeks, I can't imagine any judge not seeing this as a violation.
The other option is, you're just a troll. Any real landlord would know you can just evict them and the judge isn't going to look at permanent campers in the backyard as anything but tenants.