These instructions are from an OpenGarden.org forum posting by RoyK.)
Version: 8.05.1 and higher
First, create a template "Template:NoComments" and put some text there such as, "Comments are disabled on this page".
Then on the page you want to disable comments, type in:
{{wiki.template("NoComments", nil, "comments")}}
You'll also need to update LocalSettings.php:
$wgTargetSkinVars[] = 'comments';
This is new functionality inside Deki Wiki which allows you to overwrite skinning variables from inside the content - what you've done here is inserted the "NoComments" template inside the skinning variable "comments", thus overriding the default content that's loaded.