I would personally alert your competitor before disclosing anything the public.
As far as putting them out of business, I wouldn't be so certain of that outcome. For one thing the matter of PCI compliance is between your competitor and their payment processors. If you figure out a way to report them, their processor may just give them a chance to fix it and it will all be handled quietly. Or it could result in an audit, fines, their processor dropping them, etc. Those are all bad for the competitor, but not necessarily something that will put them out of business.
Posting on a public forum has potential to scare customers if there is such a place to post your message for your niche where "everybody" will see it. But there's a lot of ways for that to backfire as well depending on how you handle the posting of the message.
As far as putting them out of business, I wouldn't be so certain of that outcome. For one thing the matter of PCI compliance is between your competitor and their payment processors. If you figure out a way to report them, their processor may just give them a chance to fix it and it will all be handled quietly. Or it could result in an audit, fines, their processor dropping them, etc. Those are all bad for the competitor, but not necessarily something that will put them out of business.
Posting on a public forum has potential to scare customers if there is such a place to post your message for your niche where "everybody" will see it. But there's a lot of ways for that to backfire as well depending on how you handle the posting of the message.