Psychiatric Commitment
In most countries, a person who can be shown to be a risk to themselves or those around them can be committed against their wishes into a psychiatric institution for assessment and/or internment.
Realistically, this rarely, if ever, happens to adults as a result of body modification. Even in cases where the individual performs a castration on themselves and ends up in the emergency room, if they don't believe that you're fundamentally suicidal (and these are usually desperate attempts at life, not something suicidal in the least), the worst they can do to you usually is hold you for 72 hours; at the end of three days they either have to let you go or commit you for another 30 days (note that this varies from region to region).
However, if the individual is a minor, both the state and the parents have a great deal more power to commit them, often without even making a case that the individual is a true danger to themselves. It is not unheard of for parents to send their children to psychiatric institutions (or dangerous boot camps) simply for having a body piercing (or a funny haircut, or whatever). In some cases these false imprisonments have lead to long term depression and even suicide.
Parents, let me make one thing very clear: piercings are a form of expression and communication. If a kid is expressing their problems and feelings through body modification, it is essential that you listen and permit this communication. If you punish them for their modifications, you will be punishing them for attempting to communicate.