First, are you able to log into those characters? If so, then enter your character password into the space when you select "Delete This character".
If you have done what I stated above, re-log and try it again. If it does not work despite correctly entering your character password in,
(Note: your character password may be different for each character and it is NOT
the same as your account password unless you made it so)
then send in a ticket at : http://www.aeriagames.com/contact/support
and state that you are having problems with deleting a character despite knowing your character password since you are able to login. They will give you additional steps from there.