Well, one way to achieve it is by changing the code in edit-link-form.php which is located in wp-admin category.
What you need to do is replace any unwanted category by nofollow. For example replace all occurences of the word sweetheart by nofollow. And you can see the option nofollow instead of sweetheart in the foem which displays relationships. You can check that in my blog at
https://www.shiotsu-used-car.com/blog
Some of my blogroll links have nofollow, but not all.
Hope it helps.
