I know this is an old thread, but figured I would leave this here in case it helps anyone else in the future:
I wanted to do something similar but I still wanted to retain the ability to notify a visitor that no operators were available (I just didn't want them to be able to leave a message.)
What I did was remove lines 41-58 from leavemessage.tpl
This removed the form but left the offline page. I also went and changed leavemessage.title in localization to "No operators available" instead of "Leave a message".
Hope this helps!