Step 1: turn off syncing. Go to the Preferences, select the Sync tab, click on the Sync with: Nothing blob. Now OmniFocus will not try to propagate any changes you make into your wife's database. That's good, because we're going to wipe all of her stuff from your database!
Step 2: open the All Items perspective under the Perspectives menu.
Step 3: do Edit->Select All to select everything
Step 4: do Edit->Delete to delete everything
You've now got a pretty clean slate, although if your wife had created any contexts or perspectives, you've still got them.
If you want to go back to the "out of the box" database, quit OmniFocus, then in the Finder navigate to the OmniFocus directory in the Application Support directory in the Library folder in your home directory. Delete the OmniFocus.ofocus file you find there, and relaunch OmniFocus.
If you want to set up syncing, you can either set up a new folder on the iDisk and adjust the path name in the sync preferences, or more easily, just get yourself your own account on the Omni Sync Server (it's free) at http://sync.omnigroup.com
and use that.