To remove any trace of evidence, remove the hard drives from computers, unscrew the covers and you will then be able to remove the platter/s (disks), depending on how many there in each drive. Pysically destroy the platters, which is easy to do, by deeply scoring them all over on both sides.
Alternatively, if you have a strong neodymium magnet, you can just wipe the magnet over the platters and it will destroy the data or after removing the cover, drill through the platters in several places and take the lot to council tip.