What's the best way to enforce the use of custom content types in document
libraries? I've googled around but can't find anything. The requirement is to
stop users using the standard content types and thereby avoiding the use of the
enterprise metadata columns.

For example, we could have a feature stapled to a site definition that
customizes the content types in the "shared documents" library, but what if
someone creates another document library? Should we create a custom site
definition with a custom list definition in place of the normal document
library? Or could we have a feature that installs a custom list definition and
hides the normal document library list type?