Remove options from an Either macro if they are true.

So, I have set a variable called $randomPassage to one of six passages (listed below). Each passage below is associated with a quest line. I have another boolean variable for each quest (e.g. $squirrelComplete) to designate whether or not a quest line has been completed. Once the quest line is completed I would like to remove the passage from the either macro... but I'm at a loss for ideas... any ideas would be mucha appreciated!

(set: $randomPassage to (either:"squirrel","BoynRock","SnowLeopard","MountPerson","MountGoat","Pinetree"))