In Glencore's situation they do not have the means to purchase the debt even at the lower valuation.
As quoted in WSJ:
But investors have become concerned about the high levels of debt Glencore’s trading arm needs to quickly buy, sell and move goods around the world. Its credit rating is two notches above junk status, and a downgrade would freeze it out of crucial capital.
Once you hit junk status a lot of institutions will no longer lend and some other credit agreements might be locked out.
with nearly $30 billion in debt (Glencore), would need to dedicate excess earnings to repaying debt obligations, the analyst said.
So basically they do not have earnings to purchase the debt back if they wanted to and since they are near junk status they won't be able to refinance as the terms to refinance would be worse than keeping the current debt.
As for 'are there actual rules against doing so': There are not laws against refinancing most debt. However, I am sure a lawyer would know more about the actual legalities of the situation.
The problem lies in people don't loan money at good rates to people who need it.