BuddyPress.org

@nickaster “What if I want to get rid of buddypress?”
– deactivate BuddyPress and BP-dependent plugins at the same time. Then go to database and delete bp_ … tables (btw, backup database before doing this)

I haven’t encountered that issue about not being able to edit theme files at all. It would help to know the following
– what WP and BP versions are you using?
– what theme are you using?
Activate WP Default theme (in WP 2.9.2.) or The Twentyten theme (in WP 3.0) and see if you can go to Appearance > Editor and edit file.