Not allowing user to click once they've already done so

Hi,

So this one is really bugging me!

This is a scene where the user selects which character they want to follow. There are two characters, and the user will have to go through each characters story.  So if they pick character 1, they will eventually return to this scene and have to pick character 2. 

The way I want it to behave is that when the user returns to the scene after going through, say, character 1, they cannot select that character again.  I also want the character who has been selected to be "greyed out".  I've achieved that using the visited state.

For the purposes of getting help from you, I've put in a second blank slide to represent "going through the characters story", from which you can return to the character selection scene.

I can't seem to get this to work.  Any help would be greatly appreciated!