Comment control is an ongoing issue for many bloggers using WordPress and other tools, but now there is a plugin that will help to solve this problem, atleast for controlling opening and closing comments and trackbacks called Extended Comment Options.
This section gives you more control over discussion options. You can choose whether you want to open/close comments or pings, or both. You can also choose whether you want to change the default setting for new posts.
In the “Which posts?” section, you can choose which groups of posts you want to affect. The first two are for new posts and existing posts respectively. The third option allows you to open/close discussion on posts made before or after a specific date. The fourth option allows you to, for example, open discussion on the last five posts, or on posts made in the last month, or the last 2 years. It then closes discussion on the remaining posts.
Check out all the details and get the plugin at Being Mr Kenny.