[Plugin: Sociable] Cannot disable sociable in an individual post

I’m using Sociable 3.0.6 with WP 2.5.1. If I tick the ‘Sociable disabled?’ checkbox in a post or page and then click ‘Save’, nothing happens the box remains unticked. I’ve also tried adding in a Custom field of ‘sociableoff’ = true – clicking ‘Add Custom Field’ returns an ‘ok’ message, but again nothing changes. Everything else works fine – the plug-in appears in the pages correctly and I can save changes in the Settings menu.