That can be a problem with on-line betting sites [they can dispute bets]. Most pros. use bookies [when they can] or the TAB. All bet calls to a bookie are recorded and can't be fiddled with as are TAB calls. At least you can win as much as you like with the TAB. Shorter prices sure, but only on the shorties.

I'd be surprised if any pros. use on-line sites. Besides, the site would quickly get rid of them by restricting their bets to pennies or just cancel their accounts.