If you've submitted an achievement to Merit's editors and it hasn't been approved yet, you can certainly cancel it. On your dashboard, under your Recent stories, you'll notice that an achievement will be "In review" until it is approved by a Merit editor. At this point, you can still cancel the achievement:
If you need to cancel an achievement that was already approved by a Merit editor, simply contact us at (518) 429-2800 or firstname.lastname@example.org. We can remove achievements in bulk for you.