If you're in charge of the game, you really have to put your foot down somewhere.

Absolutely insist that what you say is final. No arguments.

Until your GM gives in.

Rejected extra last line:

R2-D2: Careful. You could poke your splanch out with that thing.

Transcript

Mace Windu: I can't do this any more. I'll just walk the Galaxy.
Padmé: What do you mean, walk the Galaxy?
Mace Windu: You know, walk the Galaxy, meet aliens.
GM: Maybe you should play someone else, Sally. But you can't keep swapping characters every time you get bored or annoyed. That's not what the game is about.
Mace Windu: I want someone smart and wise and nice. Someone important, who knows about stuff.
Mace Windu: Like Jar Jar! Can I play him again?
GM: Well... Jar Jar is on Coruscant. I could—
Mace Windu: Ooh! I know!
Mace Windu: Yoda is almost as cool as Jar Jar. He could be coming here after Kamino.
GM: Hmmm. All right. But this is the last time you switch characters.
GM: No more Mace Windu. This part is over.
Mace Windu: Okay. From here on in you can consider Mace retired.
Padmé: So can I have your laser sword?