I don't believe it's possible from a macro. I think you'll need to use VBA
code, and call the ExitWindowsEx API.
Randy Birch has a sample at
http://vbnet.mvps.org/code/system/exitwindowsex.htm
(Obligatory warning: Randy's site is aimed at VB programmers, so some of the
examples don't work as presented in Access due to differences in the Form
model between the two products. Looking quickly at this particular example,
though, I don't believe any changes are required to make it work as
presented in Access)