According to the help article "The alarm on most Fitbit devices turns off when you walk 50 steps", and according to support, there's no way to turn off this feature.
There should be a setting to turn off the automatic alarm dismissal so that the alarm always goes off at the specified time and the user has to turn off the alarm manually.
Moderator Edit: Clarified subject
Join us on the Community Forums!
Community Guidelines
Learn the Basics
Join the Community!
Not finding your answer on the Community Forums?
Go to the Help Site
Contact Support