Why can customers still check out when I've blocked checkout?

Usually the closure isn't active yet, the cart only has always-open items, or the order isn't an online checkout. Work through the checks below.

Checks

1. Is the closure active? Start and end times use your store's timezone (Settings → General), not your computer's. Check the status on Closures. It should say Active, not Scheduled, Paused or Ended.
2. Is the mode Block checkout? The Orders column on Closures shows the mode. If two closures overlap, block checkout wins over keep selling for the overlapping time.
3. Are the items always open? Products in your Always open collections or tags can always be bought. A cart with only those items checks out normally. See How do I keep some products on sale while I'm closed?
4. Is your plan active? Without an active plan, nothing is blocked.
5. Where did the order come from? We only block online checkout. Draft orders and point of sale (POS) aren't affected.
6. Blocked at cart? We stop the order at checkout, not when adding to cart. Shoppers can still add to cart and see the cart notice.

Changes reach checkout within about a minute. If it's still not blocking, contact support with your store address and the time you tested.