B
Black Swan
It's manipulation of traders, and seems to happen even at £1 per pt, when the risk to them of taking a bet must be tiny in the scheme of things. Why not just have the platform flash up a message saying:
'Trade rejected due to company policy. Please note that we only accept business on the basis that we can't lose.'
Sorry but IMHO this doesn't happen. The platform may fail, the software may fail, human beings at both ends of the transaction fail...