?I have being trying to do the same as the OP, particularly to the Invite message.? The method described in the Wiki page that you linked to does not work, it simply fixes the message in the middle of the default message.
In desperation I believed that by deleting the invite message from the list then going to add new one I would start off with a totally blank form that could complete and lose the default message. I was wrong and to make matters worse to get the Invite message to appear in the message list I had to add something in the optional message box; even a full stop suffices.
I have yet to find a method of stopping the default message appearing; a message that doesn't suit my needs.
The OPs comment about the Invite option below the optional message box, which doesn't appear to perform any function, hasn't been addressed.
Mike Strange