How can I decline the request to help a fellow student cheat?

‘‘No’’ is a complete sentence.

I think you can tell them the truth: that the cheating is starting to freak you out; you don't like your current reputation; getting paid for answers is the next step towards being a scumbag with a test-answer internet site; and it feels weird to help someone with a class then have them say "your help wasn't good enough -- help me cheat". You may not even have to fake getting more and more agitated as you say it, which reinforces the freaked out part.

I've often dealt with an honest student talked into sending a friend a copy of their homework "just to look at". Next thing I'm calling 4 people into my office for cheating. After, the original author would ask for advice on avoiding that peer pressure again. I'd tell them to narrate recent events: they already had one heart attack being called in for cheating, and don't need another.

It's great that you do not want to do that. You can make a polite "No, unfortunately, I cannot do that." without any further justification, if the simple "No." is too hard for you.

Some people believe there is an unwritten rule that if you help them once, you help them again, and if you continue doing so, you will help them no matter what. There is no such rule, written or unwritten. There is a reason why requests are requests. You are in a position to decline them.

If you do not like to argue, then just leave it at that. Don't give reasons, do not give justifications, just say that you cannot help. Of course, one could justify the decision in that one does not like to cheat etc., but as you say, you do not want to moralise, you do not want to be told that it's all fine, you do not wish to have anything to do with that. Therefore: no justification, just, "Sorry, no."

Most likely, it will not go down completely smoothly if cheating is an accepted part of the culture, but keep in mind, these people are using you, one-sidedly. You do not owe them anything. You are already nice enough to help with revising. Take it in stride. Continue being helpful as long as it is honest and otherwise, do your thing. Uprightness is a long-term attitude, you have to ride out the occasional unpleasantry when it emerges.