"How do you get your kids to stop buying junk food?"
Basically they are going to have to learn to deal with junk food and decide for themselves that too much is not good. You can't really stop them because it is their money that they earned by working for someone other than you- unless you forbid them from working. Since they are all older, I would have respectful discussions rather than try to take away their control of this issue.
If it creates problems for you to have junk food in the house, you can tell them about your struggles and ask them to please not bring it in the house.
You can discuss your concerns about them eating too much junk food- or spending their money unwisely- and try to get them on board with more moderate consumption/saving money for other things. Maybe talk about the family budget and how you allocate so much for certain things before splurging on unnecessary items.
If they are supposed to be using the money they earn for something particular but are spending it on junk food, then I would make sure they know you will not give them money to make up for that poor choice later.