I know this will sound harsh. But sometimes the type of love needed is tough love. Keep that thought in mind as a preface for what I'm about to say. There comes that moment when you have to stand up (even as an equal volunteer) to tell someone within your community in no uncertain terms they need to leave.
If poisonous people within the community are allowed to persist they will spoil everything around them. Don't make the mistake to believe that they only hurt themselves. They affect the health of the entire project.
Lastly, never be fooled into the attempts of these individuals to pretend they are only joking. Negativity and poisonous speech is never funny. Pessimistic, biting, and cynical speech is never the right kind of humor you want seeping into your community. Excise it before it gets out of hand. If the individual fails to change then you must ask them, instruct them, to leave.