With 0.2.x, you can use the template system, described here:
http://trac.wymeditor.org/trac/wiki/Customization
But there's no easy solution to restrict what the user is allowed to edit.
I guess you rather should define editable zones in your page, so its structure doesn't get broken.
Perhaps 'Restrictive mode' will be possible with 0.3 and further versions - I'm not sure yet.