In AirPort Utility, access the manual setup options for the Time Capsule, click on the Disks tab at the top, select the Time Capsule drive, and along with the option to archive the data will be an option to Erase. You can do a quick erase, a 1-pass zero-out, a 7-pass, or if you're crazy paranoid and oodles of time on your hand, a 35-pass.
I was wondering if anyone nows how to clear, wipe all data including back-up from the time capsule