Close comments or trackbacks on selected entries.
This plugin allows you to disable comments or trackbacks on all the entries in a blog. You have several options for this. For comments, you can disable all comments, allow comments only by logged-in users, or use the per-entry setting (which is the default). For trackbacks, you can just turn them all off.
There is also a time-delay option. You can just enter a number of days and both comments and trackbacks will be disabled on all entries more than that number of days old.
Note that this plugin does dynamic disabling, so the actual entry files will not be modified. Thus, if you edit an old entry, or you will still the the checkbox to allow comments as enabled. This plugin automatically overrides all such settings.