It depends on whether the debt is legal or not. If it's a legal debt (i.e. placed at a Vegas casino in their sportsbook), the casino can take the bettor to court, just like any other debt such as credit card or car loan.
Fees. What percentage do bookies take? That question refers to the book fee that is collected from a wager. Bookies usually take around 10 percent, this is what they call the juice or the vig.