I personally have removed the drives from old computers. I then disassemble the drives. Very easy to do with a cordless drill and the correct star bit that fits the screws on the drive. Once you have the platters out of the drive they can be damaged beyond use.
As to erasing the drives so the computer can be used. As others have said there are programs that wipe the drives but you will need to reinstall the operating system. Nothing is foolproof other than destruction of the drive.
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