If a member on your team is performing poorly

And admits and apologizes for it, flaming him or her will not make them perform better. It removes any chance of them actually trying to catch up in the game. Ping spamming over them on the minimap and calling them trash is only increasing your chances of losing, not helping you win.

If a member on your team is performing poorly, and you don't want to help them catch up, then just say and do nothing. When you attack a member on your team it becomes 4v6, and that won't end well for anyone. Not saying the poor player is inting, afking, etc., anything like that. But being treated like this in a game results in trying too hard to make up for lost time, failed ganks, missed CS, etc., and thus running into plays you shouldn't have, missing out on plays you should have made, and the like.

I've had games where my top laner fed (unintentionally, of course) and went 0-5 in ten minutes, and in this particular instance, the laner apologized, tried to play defensively, and with the help of the jungler was able to catch up by 20 from farm and ganks.

I've had games where my jungler got invaded and killed in his own jungle so many times that he was behind by five levels, and my team's response was to ward his camps for him, collapse on the enemy jungler, then help him invade after we got free kills, allowing him to catch up by 30.

I've had games where my mid laner went 9-0, then made one slip-up and lost a team fight from an array of missed skillshots, then from constant flaming, tilted and ended the game 9-15.

Help your team, unless you want the loss. You can blame the "feeder" all you want, but when you tilt them even harder because you can't control your temper, you contribute very heavily to the loss yourself.