Well in the movie the said that they can reverse transactions, but it will take time to prove
In the case of the film, it would be exceptionally unlikely that the trades would be honoured anyway.
In the context of the plot that would be fine. It only had to be an issue for Bruce for a day or two for them to move forward with their plan. Which was to show his incredibly bad decision making and to make him powerless to stop them from taking over his company. To reverse the outcome of all that would have been a far more difficult thing to accomplish than to reverse fraudulent stock trades.
I always felt he had his money back by the end of the movie.
(If you're not familiar with what a clearing firm does - they're essentially a middleman who ensures that a trade actually gets completed (settled) and at the same time reduces your operational risk by protecting you against the risk of a counterparty either not-honoring a trade, or going bankrupt before the trade is completed.)
But for the Bruce part, that's easy because part of the Bruce persona is basically a dumb playboy that's rich despite himself. He has people who take care of him and if he does something on his own then it often results in something bad. Remember that he supposedly burned down his own house during a drunken rage at his birthday party after kicking everybody out.
Edit: I was wrong, settlement used to be five days, it's now three days (T+3) and has been for some time.