When a Wizard is undone in the Back Office, this runs in the background as a task, and the user who undid it is recorded against that Wizard.
1) In the Back Office menu, under Wizards & Imports, select Wizards:

2) Click View All against the relevant Wizard type, then find the completed Wizard you want to undo and click Undo Wizard:


3) The Wizard's status will change to Pending Undo while the task runs in the background, then to Reverted once complete. Against that Wizard, you'll now also see the name of the user who undid it:

This gives you a complete history of bulk changes made to your database, including who reversed them and when.
Wizards are processed in order
Wizards are processed sequentially, so an undo will be queued and actioned in turn rather than immediately.