I've sent a support ticket on this before, so I'm going to flat out quote what was sent to me.
"Players who’ve been punished will no longer earn Hextech Chests through achieving an S rank on the Rift, nor will they earn key fragments after matchmade victories. This restriction will be lifted after your punishment ends and you’ve shown clear improvement. Because it’s based on your personal behavior, there’s no set date or time when this will happen." - MoJo Jojo
I don't technically know what they view in players in order to do this, but it make take some time and understanding from RIOT to get through to players, and some time and understanding from you, yourself, the player, in order to at least attempt to be positive.