I want to just drain by adding rocks/cinderblocks and then top with soil/plants. Will that work or will rain/snow collect and flood- ruin topsoil/plants?
It will be soggy soil -- there's no drainage for it.
- Turn it into a bog garden.
- Turn it into a water feature with a pump
- Turn it into a water garden with plants that like it wet. E.g. half fill with soil and plant cat tails.
You probably can get rid of the whole plastic bit by giving it away. Remove any existing soil/plants, discover the edge, and lift it out.
Failing this, remove the bottom of the pond. This will allow the water to drain.