To stop an alarm clock on the Xiaomi Mi Smart Clock, you can perform the following steps:

- Navigate to the alarm clock menu on the clock interface.
- Search for the active alarm clock and select it.
- Deactivate the alarm clock by selecting the corresponding option. This can often be done with a simple tap or a selection in the menu.

Alternatively, you can also work with voice commands if the device is integrated with Google Assistant. For example, you can say: "Hey Google, switch off the alarm clock".