Yes, they can. This is a contract. They can ask for whatever they want. Again: They can ask for whatever they want. It's up to you how to respond. You can plead "out of the scope of my original contract," but don't expect themt to be happy. By saying "the goal post are so far apart I can’t see any of the posts anymore", you suggest that you haven't kept a written record of what they've asked for, in a form that you can point to.

Part of the game of contracting is scope management. You'll get asked to do things. You need to reflect back set of estimates that they can prioritize, and you need to keep the prioritized list visible. They'll add new stuff. Give them an estimate and let them reprioritize. At any point in the game, you need to have a prioritized list. That's how you keep the goal posts visible.

You also have the ability to counter any request that they make. In the "comment every line" case, you could counter with "comment every block of code", or "comment every function, and anything non-obvious going on within the function." I'm guessing, based on past experience, that they might not have really wanted a comment per line, and might gag if you hand them such. It sounds like these guys aren't masters of clarity when it comes to asking for what they want and following through on it.