What is the best way to get rid of milk glands in a teenage girl's breasts?

Odd question: The milk glands make up the bulk of the non-fat/non-connective tissue in the breast. Getting rid of them would require surgically removing the breast tissue. Why you would want to do this to a teenager is not clear from your post.

Breast tissue: If the problem is unwanted milk production, then a visit to the pediatrician, and then possibly a pediatric gynecologist or endocrinologist is warranted. Removing "milk glands" is not a viable option.