In the end it comes down to “don’t negotiate with terrorists”. If they disagree with the policies that’s fine, they can bring it up at a meeting. Call a meeting even, showing you’re willing to listen to their input is important. But you‘ve got to make it clear as day that there will be no going rogue if they disagree.

In the ideal world you would be their manager. Right now responsibility and accountability are separated, which is most likely causing you stress. Bring this up with your manager: tell them your plan to deal with the situation, ask for input and their backup. That way you now have a bigger mandate, and you don’t get in trouble for being tougher than you have been so far. Managing up is as important as managing down.

It might sound odd to have to lay down the law of the land, while simultaneously advising you hear them out. It isn’t. People generally want to be heard more than they want to be right. Show them you hear their concerns. Let them bring all their frustrations out. Tell them you’ll try to improve the situation for them. But also say that you cannot have somebody on the team who doesn’t abide by the policies set, and this behavior cannot continue. The content of the policies is irrelevant to that point, it’s a clear chain-of-command issue.