You’re assuming a complex strategy is needed when a simple one can easily suffice: simply pretend that you executed the trades, give the clients their profits, and keep the losses. In other words, the broker can take the opposite side of every deal and keep the spread and commissions. If, indeed, the client is trading based on no information whatsoever, then the broker wins and the client loses.
To improve this, the broker can throw out the best performing clients, since they might genuinely know what they’re doing.
I’m not suggesting that this is what any particular broker does, but it’s a strategy that works very well, at least in principle.