We also have our garbage collected by a private company. We have a limit of 6 bags per household, anything over that is left behind or can be collected for an extra fee (which must be pre-arranged by calling the office before pick-up). We rarely have more than two bags per week, but a few times a year I do major cleaning projects and really do go to the maximum number of bags. If my neighbors were dropping off their garbage at my house I would end up with rotting trash (and raccoons!), or with a big bill. I would inform the company right away that someone is adding garbage to what you put out, and leave it to them to figure out who is doing it and what they will do about it.