A client keeps asking for "one small change" after delivery: where do you draw the line?

A real situation from freelance work: a project was delivered and approved, and then the client sends a change every few days. Each one takes 15 minutes, so saying no feels petty. After a month it is several hours of unpaid work and the client has stopped treating it as extra.

The options I know: a fixed number of free changes in the contract, a small monthly support plan, batching requests into one weekly list, or just charging every time. Each has a cost, and the first one only works if it is agreed before the project starts.

If you build for clients, which of these worked for you in practice? And how did you bring it up with a client who was already used to free changes?