This may be an issue with the jre''s text box component spark uses for message input, but when hitting CtrlBackspace or CtrlDel, the word before the cursor (resp. the word after the cursor) should get deleted. This is the common behavior of text input boxes under windows and OS X (though it''s alt/option+backspace on a mac). The Spark message input box doesnt do that... It would be a nice improvement for user experience if you could add that (Especially for people like me, who are used to that keystroke)..
Furthermore, using Ctrl+ArrowKeys (common navigation-through-words keystroke) works in Spark, so it would be a nice completion