How to erase a Mach Classic 2 hard drive?

If you have an adequate screwdriver (Torx T20, super long and thin to get down to the top 2 screws on the back in the handle), you can take off the back cover. Disconnect the data and power cables from the hard drive, unscrew it from the cage and pull it out.

Be very careful not to touch the CRT as it can still have thousands of volts in the neck and deflector.

There's plenty of YouTube videos showing you how to remove it.

If you just want to erase it, you'll need the System Disk to start it up with, and use the HD Setup tool in it to erase it and re-install the base system (usually System 7).

The Macintosh Classic II is still of interest to many collectors of older computers. One responsible way would be re-use. You may be able to sell this Mac and its accessories for at least something like 100-200 USD.

As Phil0124 indicated, it should not be too difficult to erase the hard disk using an appropriate system/tools floppy (perhaps in the pictured storage box?) containing an Apple HD SC Setup utility.