Deleting all user data from Time Machine after backing to disk?
You can erase your external disk using Disk Utility. Select your disk and choose Erase. If the disk is not solid state and you want to securely erase the data, then select Security Options and choose a level of erasure.

To delete all local snapshots, you can use tmutil's ‘deletelocalsnapshots’ verb.

tmutil listlocalsnapshotdates | tail -n +2 |
  xargs -0 -n1 sudo tmutil deletelocalsnapshots