How can I hide or delete comments and sketches?

There are several ways to manage the visibility of annotations depending on what you want to do and whether you want it to affect other reviewers.

I don't want reviewers of a certain type to see our notes and sketches.

There is no way currently to conceal annotations from reviewers of any given role, including external collaborators.  However, you can use the Add To function to add the items to a new Review, which will instantly create a new instance of the item without the annotations, without consuming any more storage space.  This will be the Review you make available to external collaborators.  See Copying Items into Other Reviews for further details.

I want to temporarily get the annotations out of the way.

Each reviewer can hide the Comments pane by clicking the speech balloon icon in the upper right (or type the H key).  This only affects the individual's browser.

Sketches can be temporarily hidden for all reviewers using the eye icon in the lower toolbar.

I want to delete annotations permanently

Each text comment can be individually deleted using its trash can icon.

Brush strokes on the current frame can be removed one by one with Undo until you move to a different frame.

All sketches can be removed from the current frame at once using the trash can icon in the lower toolbar.

You can also use the Scissors item to cut the sketches from the current frame, which will allow you to paste them onto a separate frame.

All sketches and comments on all frames of an item can be removed using the Delete All Notes On Item selection from the gear menu in the lower toolbar.

If you want to keep the annotations in a backup copy, you can use the Move function to move the item to a backup Review (which preserves annotations) and then copy that back into the original review (which will have no annotations). See Copying Items into Other Reviews for further details.