CPanel Subdomain issue

[isaacl]

I have an account with a primary domain.
About a month ago, I hosted another domain for my school as an addon until they could get setup with their own account. While it was hosted as part of my account, I added a subdomain - mail.domainname.com. The subdomain did work, but there was no option to remove it in the subdomains panel in cpanel. Recently, they decided to set up their own free hosting account with X10. I deleted the addon primary domain, and they now have it in a separate account. The only problem is that the subdomain is still in my account, and although there is no option to remove it, in the subdomain panel, where you can add another subdomain, the mail.domainname.com subdomain is showing, and they are unable to add it in their account.
How would I get this subdomain deleted?
Thanks!

[Brandon]

I removed it, you should be all set now.

[isaacl]

Thanks a lot - worked fine.
In the future, is there anything I can do to prevent/fix that?