Our system bans users for a variety of reasons as it automatically attempts to detect bots. That said, there are real users who occasionally get banned by accident which is something we want to avoid. If you feel that you were improperly banned, open a support ticket and provide the email address that you registered with.
Note: if you have set up multiple accounts you cannot use the site. If you've done so accidentally, please let us know and we can migrate you to a single account.
Additionally, non-Gmail accounts tend to frequently get flagged as being invalid. If this is your case, just send us a support ticket and we can unblock your account.