It's all my fault. Playing Tutu, I'd done some bad things, and Imoen decided to leave. I didn't want her to, so used Ctrl-Q to put her back in the party. Still, she disappeared from the area.

Now I can't get rid of her. She has no form, just a portrait. Ctrl-Y on the portrait does nothing, I can't click on it, Reform Party won't get rid of her... and I can't leave the area without codes.

I don't want to have to cheat my way around the game. How can I make her leave?

You should just load your last save but if you don't wanna you could try this: Get a NPC (i suspect you have 6 Characters including the main on your team) and then remove Imoen for that NPC. I don't know if that'll work though.

(you could take the easy way out and load your last save)

Trying to add someone else didn't work; she immediately rejoined. I loaded instead.