So I've been carefully watching how high-level OTC is played, and it seems to me that it's totally permissible to go get absurd amounts of debt as long as A) nobody decides to buy you out or you can bolster your stock price by regularly buying others' stock. In practice, this means that a company can win simply by creating cash faster than they can accrue debt. While the black market penalty is very nice, it often seems not to kick in until well after the point where it should matter. By the end, you've got a monopoly, sure, but you've also got a company that has no way out of the incredible debt hole it's created.
The simplest remedy would be to prohibit people at Bond Rating D or lower from buying out the last competing company, forcing folks to prove financial solvency before assuming sole control of Mars' economy and rewarding more sustainable economic approaches without completely disabling the high-debt high-risk strategy. Sure, a monopoly could then proceed to charge whatever they like, theoretically paying back all the debt at arbitrary amounts, but that couldn't play well with the capitalists back on Earth, would it?
A more complex remedy might involve surrendering claims to the Colony for serious amounts of debt forgiveness (say, 30-40% of total debt) as kind of a way to balance out the serious amounts of greed and provide the runner-up some small window of opportunity to catch up.
Or is everything working as intended? If so, think of the children! People will believe that the path to success is to simply borrow one's way out of any financial predicament. Maxed out your credit card? Pay it off with another credit card! MADNESS