Some disused platform faces are fenced off - Earlsfield and Wimbledon on SWR for example, and someone has mentioned the former LTSR platforms on the District.
Be careful about the term 'disused'
The fast line platforms at Wimbledon are not 'disused' - they are still maintained to exactly the same standard as the rest of the station (i.e. have fully working lighting, a smooth surface, coping stones set at the correct distance from the track to allow boarding etc) and could be used at any time should engineering work or an emergency or a train defect cause them to be used for passengers to leave / join trains.
The fencing off of fast line platforms at Wimbledon, Purley, South Croydon, Norbury, etc is all about trying to prevent suicides, etc.
Platforms that are truly 'disused' include the likes of Bromley-by-Bow on LTSR side where it is blatantly obvious that they can never be used by passengers other than in a significant emergency situation due to the lack of normal platform features.