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.
Betfair charges Commission only on your net winnings on a market. You do not pay commission on losing bets. How much Commision do I pay? This is how it is calculated: Commission = Net Winnings x Market Base Rate x (100%-Discount Rate).