A non-CSS method would be to untick 'Add links to view user posts' (navadduserpostslinks) in Site administration / Appearance / Navigation.
Hmm, it sounds like you have found a bug, Emma!
At first I thought it was a bug too, but the setting specifically applies to hiding it from navigation, so it's removed from the nav block only, but remains in the profile page. Might be intentional.
Have you tried deleting your cache?