[spiderplant]

You might be turning it off before it has written the deletion to the disc. (This 'feature' sometimes lets you recover recordings that you've accidentally deleted). Try not turning it off so soon after deleting. Putting it into standby first might also help.

If they really are undeletable, then a reformat is the only solution. You can do it yourself once you've watched the recordings - see the link in my sig.