Right click on the item. Hover over the top entry in the menu (Item settings), and untick the option "Loop source."
Edit: you can change multiple existing items at once by first selecting the items you want to change, then right click on any one of them, as above.
You can set this as the default by choosing Options/Preferences/Project/Media Item Defaults/ and unticking the option to "Loop source for new MIDI items".
|