My account was immediately disabled after registration, but I didn't violate any terms / conditions. The error displayed was not very helpful, since it just says "Your account has been disabled due to terms of service violations: http://instagram.com/legal/terms/" but didn't exactly say what service violation you committed.
Other people are also experiencing this problem, where some accounts are suddenly disabled and people can't login anymore.
Instagram is aware of this issue and there is a page where you can submit your username and email for them to check:
Hopefully, your account will be reviewed and they will reverse the suspension.