It's a violation of law to use coercive tactics - such as interfering with obtaining services from another service provider - in an attempt to collect a debt. It can also be criminal depending on the tactics. Gyms are not immune and no contract can contravene applicable state law, particularly as it relates to services related to a minor. I'm all about paying bills, but there is an elaborate legal system in the United States geared solely toward collecting debts. That system works and is intended to neutralize these situations. Under NO CIRCUMSTANCES is it legally permissible to try to collect a debt by sharing information about a debt with a third party, interfering with the debtor obtaining services from another, or communicating about the debt to the debtor by any means other than highly restrictive mail and telephone contact rules. Gyms that work together are conspiring and risk other criminal and civil legal violations such as Conspiracy, Antitrust, TCPA, FCRA, Reg B, state Consumer Protection, child abuse, and stalking.
If someone does not or cannot pay their bills, withhold services. Next, send a bill in the mail. Wait 30 days for them to contest it, and then turn it over to a licensed debt collector.