UPDATE: I improved this based on some feedback in my comments.
With ASP.NET MVC it is possible for someone to try and navigate directly to a .aspx view. In general, this only leaves them with an ugly error message as Views typically need ViewData in order to work.
However, one approach that I think will easily work is to create a Web.config file in the root of your Views directory that contains the following.
We need to do more testing on our side to make sure there's no pathological case in doing this, but so far in my personal testing, it seems to work.
<?xml version="1.0"?>
<configuration>
<system.web>
<httpHandlers>
<remove verb="*" path="*.aspx"/>
<add path="*.aspx" verb="*"
type="System.Web.HttpNotFoundHandler"/>
</httpHandlers>
</system.web>
<system.webServer>
<validation validateIntegratedModeConfiguration="false"/>
<handlers>
<remove name="PageHandlerFactory-ISAPI-2.0"/>
<remove name="PageHandlerFactory-ISAPI-1.1"/>
<remove name="PageHandlerFactory-Integrated"/>
<add name="BlockViewHandler" path="*.aspx" verb="*"
preCondition="integratedMode"
type="System.Web.HttpNotFoundHandler"/>
</handlers>
</system.webServer>
</configuration>
Let me know if you run into problems with this.