If you don't want people to bet right before the bet ends, then simply reduce the potential winnings gradually right near the end.
This is already the case, which shows you still haven't read the FAQ. 
I am reading the FAQ and if the website is coded this poorly I am amazed anyone in their right mind would use it.
"What if I created a bet address but can't bet right now?
Once you enter a receiving address and are given a send-to address, you have 3 days (72 hours) to send your bet. If your bet does not make it within 72 hours then that address will be reclaimed. BitBet will be unable to send you your BTC back, as they will have probably been allocated to someone else's bet! Always make sure that you send your first bet on a created address within 3 days of its creation. "
You happen to be wrong, on both scores. Your wrongness has been happily entered into the record, we can lol at it in a few years, np.