I doubt there is any built-in way to do it (e.g., with a registry entry), but it is simple enough to accomplish with an AutoHotKey script (you can also compile the script and auto-run the resulting file in the background).
The following script allows Alt+whatever to work as normal, but Alt by itself does nothing*.
~RAlt & * :: RAlt :: ~LAlt & * :: LAlt ::
*Holding Alt down does do something: it displays the accelerator key underline if the Don’t-Show-Accelerators-While-Typing option has been set. Of course in this case, it actually is a feature as opposed to a bug.