hehe, we had this problems years ago at my parents' house, Rox. there was a two-step process that fixed this.
1 - my dad kept an eye on our front window and saw when people were coming up to the house. he caught them coming the one day, threw the door open and yelled something the equivalent of "you kids are mine!!" that scared them off, but only for a few days.
2 - when 1 fails, hide in the bushes outside your front door (as Breygon says, not available in all areas), wait for them to come, jump out and grab one to scare the crap out of them. we actually caught and figured out who the kids were that way, and the definitely never came back 🙂