From within an assignment, subjective test questions, discussion forums, wikis, blogs and journals, and grade center columns you can detach the rubric which is associated with that content. This will NOT delete the rubric, only detach it from that particular item.
From inside the Edit Assignment (or Edit Question, etc.) and under the Grading or Rubric heading, you should see any attached rubrics (they are highlighted in yellow).
Assignment window: 
Test question window: 
There is a delete/detach button to the left of the name of the rubric (it is the first of three buttons). (THERE WILL BE NO OTHER WARNING, though if you change your mind you can simply Cancel instead of Submit.) Click it.
Assignment window: 
Test question window: 
Be sure to click Submit to save your work.
From within an assignment, test with no questions, or discussion you can detach the rubric which is associated with that content. This will NOT delete the rubric, only detach it from that particular item.
Access the (assignment, test, discussion) settings by clicking the gear icon in the upper right.
Scroll almost all the way down to Additional Tools. Find the rubric you wish to detach. Move your mouse to the right of the rubric title until the trash can icon appears.
(THERE WILL BE NO OTHER WARNING, though if you change your mind you can simply Cancel instead of Save.)
Click the trash can.
Be sure to Save your work before exiting the screen.