What is an idiom applicable when someone is voluntarily participating in an action, and complaining excessively? I thought of using "don't let the door hit you on the way out", but I don't want to tell the person to leave, just want to say something along the lines of "nobody is holding you here"?
If you can't stand the heat, get out of the kitchen.
Don't persist with a task if the pressure of it is too much for you. The implication being that, if you can't cope, you should leave the work to someone who can.
But sometimes a person, especially one who never seems to be satisfied, deserves a sarcastic epithet.
If the person still doesn't get it, just tell him or her straight:
In a speech in San Diego in 1970, then Vice President Spiro Agnew used the phrase "nattering nabobs of negativism" to describe supposed intellectuals who attacked American policy....
the phrase does paint an interesting although unpleasant word picture of a self-important person nagging and criticizing everyone else.