Sounds like there are two separate issues--one, people don't know the procedure for requesting time off and two, they see your last half hour of work as a time that you are approachable.
For the first, I'd go over the requirements for requesting time off at the next staff meeting. Written requests only; it's a union rule.
For the second, I'd also start by addressing the issue in a staff meeting. Point out that you *all* want to leave at 5:30, but interrupting the money count makes that difficult for you.
I suspect that the employees are approaching you at that time because there are no customers in the building then. While the office is open, they are supposed to be helping the customers, so they don't have time or think it will look as if they are neglecting the customers if they come to you with a question or request not directly related to helping a customer.
Then tell them the new rules. While you are counting money, there are to be no interruptions unless it is an emergency. Then define emergency--fire, someone's bleeding--and what's not an emergency--vacation requests, venting about a customer. Make a sign or use the bulletin board and show them. Tell everyone you will just be ignoring them if they approach you during the money count.
You say that the rest of the staff is basically done at 5 pm, but you have work that keeps you there until 5:30. What is the reason the rest of the staff has to stay? Can you give them work to do that will keep them busy at least part of the time between 5 pm and 5:30--cleaning, restocking, a daily report?