How to prevent viewers from modifying a dashboard

I just created a dashboard and am trying to add new users with the role of "viewers" instead of "administrators" so they can't modify the dashboard. Is there a way to prevent somebody set up as "viewer" from unlocking and modifying a dashboard? (there is a lock icon at the top right of the dashboard to prevent making changes, but "viewers can unlock it to modify the dashboard).

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Joseba Celaya
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