Idiotic premade teammates who surrender when your team is leading...

Just the usual way, in the end of game screen.

I suggest Report for "Negative attitude + Intentional feeding" with a text description to explain what really happened, since it's not really feed.

And as every time a thread like this pops up, shout out at Riot: can we F-in' get the old "Assisting enemy team" back, for Joe's sake... That's much more generic and useful as a reason to report.