Reasons include test bans, excessive spamming (e.g. a spam link), excessive inappropriate projects, studios, comments, or posts, excessive threats or unfriendly statements), promotional or inappropriate usernames (e.g. BuyItAtExampleStore or TheRealKaj12345) or User Icons, repeated addition of inappropriate Profile Page content, hacking other accounts, and accounts created to evade blocks.
If a user was not following the Community Guidelines after repeated warnings, their account will be banned from the website. Sometimes it is more severe, with an entire IP Address or range being banned.