When To Kill A Character

Ever have a character you don't know what to do with after a certain chapter? Maybe they were an integral part of the beginning, but lost their purpose as the story evolved. You can't go back and delete them without having to change too many details that are necessary, but you can't move forward with them either. What do you do?

So far I've come up with the following:

1. Have something traumatic happen to them to increase the conflict of the story (i.e. accident/coma/terminal illness).
2. Have them move (make up a short back story of a long lost relative that needs them).
3. Kill them off.

I write characters. I should know how to delete them. How do you get rid of your characters when they can no longer be written?