I am not convinced from the way you have worded this that the agent did anything wrong.
You say 'prospective' buyers who signed a purchase agreement. You did NOT say you accepted the agreement and the agreement was affirmed by lawyers for both sides. If those things did not happen, then there was no contract and the buyers can change their mind.
You also did not say who brought the buyer. Did your agent bring the buyer or did another agent bring the buyer? It's important. Because if another agent brought the buyer, is representing them, that agent has every right to use the lockbox without your agent being present. That's what lockboxes are for, to allow for more efficient use of EVERYBODY's time. If that other agent allowed their buyer in by giving them the key or a code, and did not accompany the buyer, then you have an ethics complaint against THAT agent, not yours - but if the contract was not finalized before the 'smell' was discovered, it doesn't matter - the buyers can change their minds and walk awy for any reason, provided their attorney has not already approved the contract on their behalf.
If your agent brought the buyer and the buyer went in unescorted by your agent, then you have an ethics complaint against your agent. The local real estate board can deal with that. You don't have any grounds for a lawsuit against the agent if the deal died. You may have grounds for a lawsuit against the buyer, if the contract was valid and they breached it. That's for YOUR lawyer to look into.
When I was selling, there were no electronic lockboxes. All agents were given a master key, by their brokers, that would open any lockbox used in the MLS. If you left that brokerage, you turned it in. I had a key and I showed many a property using that key, and I also used lockboxes on some listings. And no one ever had a problem. No one was stupid enough to give their key to a potential buyer.