Contrasting with the in-game suspension systems, the Boards suspensions aren't actually tied to specific ban codes. As mentioned below, this was likely a pro-active permanent suspension for posting spam/scam links. If you contact Riot Games Player Support through the Support Portal, you may be able to appeal your suspension.
As always, it's best to be careful when clicking and sharing external 3rd party links. In order to keep your account secure, it's a good practice to review the Protecting Your Account Riot Games Support article.
For more information on the Boards Guidelines, check out the Universal Rules on this link:
http://boards.na.leagueoflegends.com/en/c/player-behavior-moderation/4LgZ2EwM-universal-rules
If you have any questions or concerns, don't hesitate to leave a comment. Thanks!