Removing anonymous event handler

I have the following code where SprintServiceClient is a reference to a WCF Service-

public class OnlineService
{
    private SprintServiceClient _client;
    public OnlineService()
    {
        _client = new SprintServiceClient();
    }

    public void AddMemberToTeam(MemberModel user, int projectId, Action<int> callback)
    {
        _client.AddMemberToTeamCompleted += (s, e) => callback(e.Result);
        _client.AddMemberToTeamAsync(user.ToUser(), projectId);
    }
}

the problem is that every time AddMemberToTeam is called it adds another callback to client.AddMemberToTeamCompleted

i.e the first time AddMemberToTeam is called the callback is called once, the second time AddMemberToTeam is called the callback is called twice ect.

Is there any way to remove the eventhandler from AddMemberToTeamCompleted once the eventhandler has been called or use another method which takes in the callback?