It is possible to achieve what you need (to disable editing dashboards for non-admin users):
By default the dashboards has permissions "Word Writable", which means that anybody can add/edit widgets on the dashboards. You have to change permissions to the dashboards to "World Readable" on the Explore / Content page (you can filter all dashboards - by type; and the edit button is at the bottom left side). After all the dashboards are set to "World Readable" they are not editable by non-admin users (it is not possible to edit widgets or add new widget). More details on the following screenshot:
