With my older kids, I would tell them if I needed to start to count, I would be counting the amount of favourite toys of theirs I would be taking away from them for a week. I would just give them a warning and then if they didn't co-operate I would say "Okay, 1. That's 1 favourite toy gone for a week" and no matter what, I would stick by it. They soon found out that I was serious.
That worked for us for a long while.