The Pentax (and your regular Windows) format process doesn't erase the card. It just clears some headers that listed the file start address, and allows that space to be overwritten. I have tested and can recover all the pictures after a format.
Find a friend with a Canon camera. The Canon's have a low level format option that will fully wipe out the picture files, and the above programs won't be able to recover anything. Really easy and fast, as long as you both use SD cards.
There are utilities like
Eraser from Sourceforge. It is designed to make multiple pass writes of data patterns to hard drives and is good enough to foil CSI type forensics experts. If set up to just make one pass, that's good enough to scrub files off a drive that you want to give away. You can run it on a drive and it will scrub all currently un-used space too. It will also work on card reader contents. It takes some tech skill to install and set up. After that, "erase" become one of the options like "copy", "delete". etc when you right click on a file. Like anything else with such capabilities, one could probably screw up with it and crash a Windows install. Just wanted to point out such tools are available, and the sourceforge people seem pretty reputable.