How are they getting in? Do you have a cat flap? Too bad you can't install some sort of paw-print ID verification system...
My mother has a dog-door for her small dogs and the neighbor's dog used to come over and pee in the house. My mother attached a note to his collar, and the next time he wandered over he was wearing a diaper!
OMG that is hysterical!!! We have many open windows, and are lazy to shut them in summer especially. I like the ID verification idea.....hmmmmm!