No need to convince them to leave. Every position, volunteer or not, comes with a stipulation that if the work isn't done well or if their presence there causes a disruption, then a superior should tell them that their service is no longer required. Basically fire them.
Sometimes, you just have to be honest with people. No one wants to hurt someone's feelings, and you should use some sensitivity, but you must be firm and explain that you cannot work with them any longer. There is simply no alternative to confronting this person and telling them that they are not helping your cause.
I am sure you could do some thing along the lines of telling them you need to give another person a chance to volunteer and then provide them with an alternative position or ask them if they will help at another, more needy, volunteer position. They may be happier and feel more needed. Sometimes that is all a person wants.
Give him less tasks to do. Relegate him to tasks that don't disrupt the functioning of your organization or hinder the productivity of your other volunteers. Alternatively, you can just be direct and tell him it's not working out.
