Ahm, good question 
I was never Comp banned so I do not have any personal experience in that department.
However, this was the “newest” post that I could find regarding leaver penalties (if someone else has any current ones, please correct me):
As far as I understand the leaver penalty goes down once you stop leaving matches. Also, I do believe you will have to wait for the next Season to be able to play again.
All in all, do not leave matches or if you like doing so then play QP or other non-Comp related modes.