By being completely new. That's how. If you're level 25, why don't you set an example for them? Showing them toxic behavior (i.e calling them faggots, excessive pinging etc. etc.) isn't going to better the (already fucked up) community. Since you're at such a high level playing bot games anyway, try to lead by example.
As for the TF feeding:
It's a bot game. The odds of losing are so astronomically that low that even if you feed, you're still gonna win. Your game isn't even close to being ruined.
A ruined game is carrying your team to a Baron steal, and taking down their Nexus turrets only to have your team drop the ball after you net a quadra and die. Leading to a loss. That's a ruined game.