I learned this from Raine so this idea totally belongs to her. This is faster than the method Vivi posted.
1. Go to the character selection screen
2. Pick a character and press "del" button
3. When it asks you for your e-mail, DON'T WRITE ANYTHING. Just leave the space blank and press enter.
And voila, the character is gone.
Now then, that is a pretty damn big problem, don't you think? The client executable should be patched so that isn't possible anymore or else someone might delete others' characters just by getting their password.