Marvin lets you turn tasks into recurring tasks and projects into recurring projects by clicking its "Edit Task" cog icon, and then choosing "Make Task Recurring" at the top. But how do you undo this and make the task no longer recurring?
First find any instance of the newly created recurring task/project, and click its "Edit Task" cog icon. Then click the "Clone" icon (also at the top, next to the "Make Task Recurring" icon. This will create a copy of the task/project which isn't tied to the recurring task parent. Then you can delete the parent recurring task/project in ☰→Recurring Items to clean up.