There's two different ways you can remove someone from your 1-on-1 category.
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First, you can navigate to your left control panel in the "home" view. If you hover your cursor over the user you wish to remove, you will see a vertical 3 dot menu appear to the right. If you click this menu, you can then select "Remove from home panel" from the drop down menu that appears.

The next way you can remove someone from your 1-on-1 category is within your 1-on-1 with them. In the upper right hand corner there a little person icon button. When you click on this icon you will see a drop down menu appear where you can then select "Remove from 1-on-1s".

They will now will be hidden from view. 

(However, if you ever need to access those notes, you can search for them)