Can Mark delete a campaign, ad set, or ad?

No, and that is on purpose. Mark can create, edit, and pause your ads, but it cannot delete campaigns, ad sets, or ads. It is a safety feature, so you never have to worry about your work being wiped out by mistake, whether by you or by the AI.

Pause instead

To stop something, pause it. A paused item stops delivering and stops spending immediately, and you can turn it back on whenever you want. In practice, pausing gives you the same result as deleting, since the ad no longer runs, but without the risk of losing it forever.

If you truly want it gone

If you really want to remove something from your account entirely, you can do that yourself in Meta Ads Manager, where deletion is available. Mark deliberately leaves that final, irreversible step in your hands.

This one limitation is a big part of why Mark is safe to hand real campaigns to: nothing it does can destroy your work.