Can someone be permabanned for using custom skins?

On a Facebook group that I am a member, someone posted that they have been permabanned for using a custom skin.

Almost all members of this group berated him for supposedly doing something that is against Riot's rules, however, as far as I know using custom skins does NOT violate Riot's terms of service. I even searched for proofs on Google and this led me to this: http://forums.na.leagueoflegends.com/board/showthread.php?t=2436518&page=17#post27906871 and this: http://boards.na.leagueoflegends.com/en/c/player-behavior-moderation/7JBfAA5o-perma-suspended-for-use-of-custom-skins

I am still not completely sure if Riot prohibits the use of custom skins.