When setting up rules in your Settings, under Policies > Rules, you'll be able to set a return window per return option (click on "Edit" next to the return option):
This return window will block customers outside of the window from returning their items. By default, they'll see this message:
You can edit this message with our Advanced Customization features. Reach out to us so we can activate those for you!
If you want your customers to always be able to place a return, but have them be approved or rejected by your customer service team, then you can also choose to set an unlimited return window (set to 0), and manually approve your returns.