Some customers have report models that are large enough they do not expect any user to ever request the entire model, and would actually prefer to disallow that option, and require them to choose a perspective instead. There is an undocumented and unsupported feature that provides this behavior in the current release. It is used by Analysis Services when generating a report model over a cube.
The simplest approach is to use the SOAP API's to add a custom property to the report model with name = "MustUsePerspective" and value = "true".
If you wish, you can add a similar custom property to the report model within Model Designer, set the property data type to Boolean, set the namespace to "http://schemas.microsoft.com/sqlserver/2004/11/semanticquerydesign", and whenever you publish the model the corresponding server property will be added automatically.