What can you do to prevent cavities from getting bigger?

Cavities. If they are small, just beginning you can reverse them with proper home care. Larger cavities require repair, followed by proper home care and usually a change in diet. If you are getting a lot of cavities, you probably have multiple exposures to sugar. You need to limit the frequency of these exposures. Ie its better to chug a big gulp soda than to sip it throughout the day every 10 minutes.
See a dentist. The dentist will clean out the cavity and restore the tooth.