Check out the templating documentation at
YOURSITE/doc/acs-templating
You probably need to remove the <master> tag from your ADP templates. That will suppress the openacs template around your content.
If the content is in a Tcl variable, you can also just return it from the Tcl script like this:
ns_return 200 text/html $var
or
ns_return 200 text/xml $var