Old paperwork

We have got a lot of old paperwork that has been left here by clients, many of whom have gone out of business. I have no idea whether these people are at their addresses. We have rung them all but noone has come in to collect their paperwork. We have no space left now for current clients. I am loathed to pay postage on these items, particularly as they may not be at the same address now. Can I write and say that in X days it will be destroyed if not collected? Surely I should not have to pay for archive space myself at a local storage unit in oder to store other people's things. What do others do?