Confirm it first
Look up the subdomain and check where the CNAME points. If the target is a hosted service and that service returns its own unclaimed page, the record is dangling. If something normal loads, it is fine and no action is needed.
Remove it
- Open your DNS provider, which is where your nameservers point rather than necessarily your registrar.
- Find the record for that subdomain. It will be a CNAME pointing at the external service.
- If you no longer need the subdomain, delete the record. That is the whole fix.
- If you still need it, point it somewhere you control first, then remove the old target. Never leave a gap.
- Check nothing else references that subdomain: printed material, email signatures, partner documentation, redirects.
Why it matters more than it looks
With several providers, the name you released can be claimed by another account. Whoever claims it controls what is served at your subdomain, on your domain, often with a working certificate. The practical uses are phishing that is genuinely hosted by you, and access to anything your main site scopes to the parent domain.
Stopping the next one
These appear whenever a hosted service is trialled and dropped, which is why one cleanup does not settle it. The habit worth building: when you cancel a service, delete the DNS record in the same sitting. The cancellation and the record are separate actions and only one of them is on anyone's mind.