I have an Armitron (Model: AMDY6003) from Academy Sports. The way I turned off the alarm:
1. From the main screen (displays time), press and hold the lap/reset button (bottom right).
2. While the lap/reset button is still pressed, press and release the start/stop button (top right).
3. The little graph/icon showing the alarm is set will now disappear.
It worked for me. I hope it works for you too. I don't have the user's manual so just tried hitting buttons.