I have two forms: Managers and Players

I have a third form called ManagersPlayers that has the lookup to ManagerName from the Managers form. It has a subform that contains a lookup to the Players form.  This works fine. I select a manager, add a player. 

Now, it gets fun(ky). I have a field in the Players form called "Available" as a drop-down with YES/NO choices. By default, all players are YES available. What I want to happen is when a player is added to a manager via ManagersPlayers subform, I want that player's Available to change to NO so that the player cannot be chosen again. (I still have to workout how to hide choices based on YES/NO, but I'm taking one step at a time).

Any ideas? I tried the script wizard/workflow thing, but I have no idea what to do in there.