There are several reasons that a bacteria may stop growing.
1. Bacteriophage- viruses that destroy bacteria
2. Immune System- when in a host, host's phagocytes (white blood cells) engulf and kill bacteria
3. No food- such as bacteria grown in the agar of perti dish, if the food runs out, the bacteria will be forced to cease growing and soon die if other means for acquiring energy do not exist.
Hope this helps you out! :D