How to deal with feeders?

How does one deal with them?
They never listen.

Worst case scenario: Our team has a Morphling who doesn't listen and morphs full agility then feeds

Depending on if they are new to the game or trolling. In the case of a new player, I would explain to the player that X is the best way to maximize your use with X character. Give them a few tips like where to place the wards or one way to gank or even what ganking and wards are or suggesting that they try a few bot matches then co-op bot matches. Maybe even play one or two with them, that's the best way to deal with a new feeder also helps to build a better community. In the case of a troll player. Not much can be done other than doing your best with a teammate like that maybe reporting him if you are sure that he is just running into their throne. A good rule that I like to follow is always assume that a player is new. Some new players go straight for matchmaking. Anyways those are my thoughts.

Act like you don't care and keep playing. The reason they are feeding might be because you annoyed them and they want to annoy you, too.

So, if you act normal and don't give a dam about it, they might stop at a point.

Happened to me, the guy stopped at 0/26 or something. Better than 0/30 I guess.

If you can't carry them? Smoke, quelling, hide in trees. gg next.

There isn't a way to deal with them because if you nag at them obviously they wont listen. Just accept the fact that you can't do anything about it and continue playing how you usually do. Took me a while to get over the fact that i turn into a tomato when i play with people who troll/feed constantly. Just be nice and they'll probably apologize at the end of the game or ask you to be their mentor.

Tell them how to improve, if they don't listen or react negatively, not much you can do other then wait for your Ancient to go down unless the rest of your team can take the slack.

If they do react positively, show them what they are doing wrong and be active at monitoring them. If they are in a vulnerable spot with a few heroes missing, tell them to get back, stuff like that.

Some people are new. They don't know why they keep dying so easily. The best response is telling them what to do. But they become bitter from dozens of games where when they die twice they are pummeled with phrases like "fk you noop, you feeder", or "delete dota and install tetris", or "cyka, noob feed". This creates in them a air of resentment towards the community, and its inhabitants. But, they strive on, trying to survive in the hell that is the Dota Community, growing more and more on edge, growing into the very thing they are trying to survive.

In the end, they start to lose it, and when one guy tells him something normal like "Don't morph full agility", they snap, replying in various swear words, lost to the punishment taken throughout their new Dota career. And, after this, the circle of doom simply descends into a dark crypt of anguish and hate, as flame wars start, muting, reporting and eventually intentional feeding becomes the order of the day.

So, before you flame that guy, start off with a helpful hint, and see if he might not just be new to Dota.

tl;dr : Never flame a feeder, or you just make the Dota community worse. Help, or accept. No point in typing in all caps about it.

sometimes there is just nothing you can do. Win some lose some