For this reason alone, Betfair has no reason to ban or close your account when you should win. This is what Betfair first became famous for; peer-to-peer betting. Whilst the Exchange account won't be closed for winning, there is an additional commission rate.

If the matter is not resolved to our satisfaction, we will restrict or permanently close your Account. Any balance in your Account at the time of any closure by us will be paid back to you, except where: you have engaged in illegal activity. we determine that you have participated in any Prohibited Activities.