Neither ArcGIS OnLine Web Mapping Applications nor Web Maps allow authors to limit how far users can zoom in or out. It is possible if there is a cached base map, but not possible otherwise. Setting scale dependencies and limits on REST services and layers does not negate this need. That will stop the layers from being displayed, but still allow scales that may cause users confusion, frustrations and problems. Setting a default scale and extent will also fail to address this. This function was possible in an older ESRI platform, ArcGIS for Flex, by specifying LODs.
Example 1: A global map showing total population by country allows a user to zoom in to 1:1,000. At that scale the user will most likely be deep inside one country's polygon requiring them to have to pan and zoom many times to see another country to compare to.
Example 2: A map shows child poverty rates by Census blocks to three counties in southwest VA. A user may zoom out to 1:73,000,000 to see the entire country. At that scale, the displayed Census data will not be visually obvious and hard to find again.
This is similar to another idea submitted in 2013, however, this request is simpler. I am just asking for scale limits without a lock on panning, a harder feature to implement.