If you find a player to be inting or otherwise feeding/throwing, make sure to report post-game lobby with as many details explaining the incident as possible. After that, it goes to Riot and after enough reports the account gets flagged for review. Remember, you only get the notification your report gave the player a punishment if it was your report that triggered it. If you report, then someone else in the future reports and the account gets punished, you won't be notified.
What I will also say is that not building SS isn't inherently something you can report someone for and get the punished with. If they are outright running it down mid, feeding, and purposely throwing then yes, but not buying SS and having a bad KDA can also be a simple case of a bad game. The key is consistency, if he does this constantly whenever getting autofilled, and he is trolling, the system will do it's job and punish him accordingly.