How to force OS X Notes application to lose formatting when I paste text from clipboard?
You can use OptionShift&#x2318;-V to paste and match the current style. This removes all previous formatting, links, etc.

If you would like to make this the default action when pressing &#x2318;V, open System Preferences -> Keyboard -> Keyboard Shortcuts, and select Application Shortcuts from the list on the left. Then, hit + to make a new shortcut, select Notes.app from the dropdown list, enter Paste and Match Style exactly in the Menu Title box, then hit &#x2318;V with your cursor in the Keyboard Shortcut box. After hitting Add, you can flip back to Notes.app and click on the Edit menu to see that the shortcut has been reassigned.