If you want to constrain map at a given extent, you can subscribe to ViewpointChanged and check whether the current viewpoint is still within the maximum extent you want to set it to.
var extent = MyMapView.GetCurrentViewpoint(ViewpointType.BoundingGeometry)?.TargetGeometry?.Extent; //current extent
You can use GeometryEngine.Equals(extent, _lastCheckedExtent) and GeometryEngine.Within(extent, MaximumExtent) to ignore ones that you've already checked or fall within set maximum.
Otherwise, MyMapView.SetViewpoint(new Viewpoint(MaximumExtent)) to set viewpoint to desired extent.