You can only bet during warmup.
Bets are hidden.
Payouts are based vs %'s, so if 100% of bets are on traitors, nobody gets anything.
Maybe if you bet on a team that's not the one you're on, the trade is nullifed so there's no possibility of ever throwing the round.
It might be overboad, but it's the only thing I can think of to fully stop people manipulating the betting system, other than kicking/banning people for throwing for yomocoins.