How to get plaster of paris out of a plastic container, and how do you dispose it?

My dad made me use plaster of paris for my papier mache project. I told him that it was a bad Idea, but he wouldn't listen. It hardened in the container almost as soon as I mixed it. It is stuck to the container, and it's hard. It also says not to put it down the sink as it will clog the pipes.