Consequence Severity for Leavers

We all know that leavers are an issue in League. So why doesn't Riot make it so that leavers get a days ban instead of a warning message? or anything meaningful that could actually change behaviour. A message after you leave doesn't do anything. Leavers will still leave. If there was an actual consequence to leaving, leaving wouldn't happen as often as it does.

A tap on the wrist isn't a good method of changing behaviour.