Can I undo something Mark did?

Yes, and because Mark cannot delete anything, your work is never wiped out in the process.

To stop something spending

Pause it from Manage Ads, or ask Mark to pause it for you. Paused ads stop delivering and stop spending immediately.

To change a value back

Just tell Mark. For example, "put the budget back to 15 a day," then approve the change. Since Mark keeps a clear record of what it built, reverting is straightforward.

Accidental builds are harmless

Because new campaigns are created paused, a campaign you did not mean to launch never goes live on its own. You can review it and simply leave it paused.

Why there is no delete

If you want something gone from view rather than paused, note that deletion is intentionally left out for safety. Pausing achieves the same practical result, since the ad stops running and stops spending, without any risk of losing your work.