Trap and dispose is the only sure way. Don't get experts involved (had one trash a rental house after dropping down the chimney, ripped down blinds and curtains, shat on furniture, the works. Agent called a possum remover. Upshot was the possum had its damaged paws treated, got a few days in animal shelter, and was RETUREND TO THE PROPERTY cos that's the law. I got the bill. Don't know if this represents justice or not but a couple of days later found a run-over possum on the road outside the house, and I've never had a possum problem there since). It's interesting that diesel exhaust seems to be non-toxic to possums, but petrol exhaust isn't.
