Are you sure what you have is a cavity or could it be some staining? This is a hard judgment for dental students to make as they are learning to diagnose cavities. If it is a cavity, it is a bacteria infection. The only way to stop it is to remove the bacteria or get them to stop. This is normally what a dentist does while he/she is removing a cavity as you cannot stop the bacteria without cleaning out the tooth. In a pinch I would say fluoride rinses and step up the brushing and flossing game, but this is not a permanent solution. I would also say to get a free consultation or second opinion as these are normally free where I practice. Hope this helps.
My best to you!
William F. Scott IV, DMD