Most of my personal experience is with J-1 pimps. These "firms" import seasonal labor and directly collect wages from the hiring company. The catch is as old as markets, as they usually debit housing, food, and fees that are grossly outside of true cost, but allow for just enough origin country adjusted income to still make sense for students to keep signing on. I have dealt with these firms from Eastern Europe and South America, with both operating on the razors edge of indentured servitude. These exact same practices dominate the H-1B program.
The most effective change to any of these programs, outside of just volume, would be the direct to employee payment requirement or minimum percentage. That change would force the either higher salaries in order to maintain the labor firms margins, or the labor firms would have to eat the cost to keep the prices down. Enforcement would have to be extreme. As the nature of the relationship between the firm and the visa holder is already indebted, and there would be lots of opportunity for harassment and coercion of the visa holder to fork over more of their paycheck.
Addition: Another solution may be a visa marketplace that the government would run, to connect employers and visa seekers directly and eliminate the middle man.