Not everyone who sucks or does badly is a feeder. I think what OP is talking about is the solo top who dies a couple times early and then says "The hell with it! Let's see who I can annoy for the next 12 minutes."
I see it happen a lot where a usually good player dies early to some poor moves and then just gives up on the match. There are also the ones who go into the game trying to mess things up early.
A good example is a Tryn we had who was going solo top. Our WW (jungler) went to blue buff and got help from Tryn. But instead of tagging and going away, the Tryn stole the buff and ran into Baron. The entire match was over in the first 2 minutes. People started leaving right at that moment.
These are the people that need the banhammer ASAP and Riot could do a little more to focus on these people. I will agree though that Riot is really good about banning them once they get enough 'proof'.
Just report exactly what happened and urge the other players to do the same. Most enemy teams will gladly report players for being like this with no issue