[tbo]

I tested it as Tom suggested. The problem is for me that those calendars are shown with dashed lines around them so I cannot delete them.
I have messed around with that when trying the same thing months ago already. Since I recently rearranged my Google calendar, and integrated it into iCal, I am now able to move working hours around straight from iCal without making it public in the first place. But for cleaning up, I want to get rid of those old blocks in OP.
It would make sense to me if I had deleted all Normal Hours for the resource assigned some tasks and then trying to delete extra hours (some sort of warning would show up: "This resource has tasks assigned to it. All assigned tasks will be unassigned."). But is not the case. I deleted the subscribed calendar from the menu - and the blocks are still shown for the task (with a dashed line around it).