When selecting an item on the Newsletter, you can navigate to the top toolbar and on the right side select the Padlock Icon. This will give you a few options for locking down the asset.
Size and Position can lock the width, height, rotation, and X and Y coordinates of the item on the Newsletter.
Style can lock options such as alignment, spacing, columns, tabs, and other text styling options, as well as borders and opacity on images.
Content can lock bullet-points, text links, highlights, and images.