From Red Hat Apache Guru Mark Cox, via Drew Meeks at Red Hat:

Adding

Header add MSSmartTagsPreventParsing "TRUE"

to httpd.conf should add the response header to every page served.

Mark and Drew haven't tested this, but I pass it along. You can test the efficacy of this approach at http://209.204.184.53/, which returns both

MSSmartTagsPreventParsing: TRUE
X-Meta-MSSmartTagsPreventParsing: TRUE

as response headers, the latter since X-Meta-Foo in the response headers is interpreted by some browsers as equivalent to a <META name="Foo"> tag. http://209.204.184.50/ returns the same content without the above headers.

Requires that mod_headers is installed on your copy of Apache: See http://httpd.apache.org/docs/mod/mod_headers.html. You can alter the scope of this by placing the "Header" directive inside a VirtualHost, Directory, Location, or Files directive.