Step 1: On your "You" page, click on "Plan 1:1s"
Step 2: Click into your 1:1 relationships in the upper left hand corner. From here, click into the 1:1 relationship in which you would like to delete.
Step 3: Once you are in that 1:1's profile, click into the white "Manage settings" button in the top right corner of the screen
Step 4: Scroll down to the bottom of your 1:1 relationship settings and toggle the 1:1s button OFF. Don't forget to save your settings
Note: You must be the individual that created the 1:1 relationship to delete it. If you turn that 1:1 relationship back on, all of your 1:1 history with that individual will be restored.