Like odeon said... hammer works.
Turn it over and hit it hard a few times on the center hub until it sort of bends in, which is usually the quickest way to make it unreadable.
...or take it all apart. There's usually 2 really strong neodymium magnets inside.
Taking just the bottom board off is not enough though, as the board can always be replaced with one from a similar drive making it functional again.
I take them apart...the bottom boards have scrap value and go in a bin with the other boards & ram from the computer, magnets have some resale value in lots, hd case is normally aluminum and goes with the aluminum.