Violation of rights,i called the cops to remove people from my home for trespassing, & they let them stay!?

ok so i let my brother in law & his wife stay with me a couple of nights here & there cuz they were sleeping in their car,i also let them store their things in my basement,but they were all bagged & boxed.One night they waited until about 11pm & snuck through my backdoor & tried to stay in my basement,assuming i was asleep.so i called the cops & asked them to remove them from my property for trespassing...the cop got here & told me that i had to have an eviction notice to get them to leave & that they could stay in my home for however long the eviction notice stated.now they are of course saying that i agreed to let them live there so they wont get in any trouble,but they have no proof whatsoever & from my understanding,even with a verbal contract you have to have a door key,lease agreement,mail coming to the adress or something. this cop simply took their word for it & left them in my home & im wondering if theres anything i can do about it because i definately feel violated.