In the NBM project properties Build->Packaging panel, there is a "must be
installed globally" checkbox.  However, as the project lead explained after
chastizing me for being so stupid as to actually check that box (what was I
thinking?):

"Under what conditions is it okay to have a globally installed module?" -
exceptional conditions. It is never desirable, but in a few cases it is
impossible to avoid. Generally for modules which participate in the core startup
sequence before the module system is ready. Also for the harness pseudomodule
since the files it bundles in its private cluster have a distinguished function
for apisupport.

Because the number of potential modules that need this option set is so small
and because it is easy to set this property manually, this checkbox should be
removed from the UI.  

An alternative solution is to look up the module author's NetBeans login name,
and send an "xkill <username>":

   http://linux.about.com/od/funnymanpages/a/funman_xkill.htm