How to deal with a fed enemy?

that changes depending on who your teammates are, how far behind they are, whether or not anyone else can assist you, but most importantly, WHO the fed person is.

you would deal with a fed karma far differently then a fed nasus.

kill him

One fed person on a team means the rest of them aren't going to be that much of an issue. See if you can try and team-gank the fed one. Even if you lose someone in the process, the following 4v4 will be much more even.