I've just had a chat with admin at B/O (as you know this site nothing to do with us at Boatsandoutboards4sale) regarding a different subject but this issue came up.
Apparently they are part of a larger group of website owners including autotrader, ApolloDuck etc trying to combat this issue. They say they have a team of 20 people dedicated to trying to route out the scams but freely stated its a sign of the times that so many scams are present and it's impossible to prevent. They also claim to remove any reported advert which is shown to be a scam. In fairness they've always removed the ones we have reported too.
I guess thats about the best we can hope for.
Peter @ Boatsandoutboards4sale ~ firstname.lastname@example.org
~ 07930 421007