It's not difficult, keep in mind that you may derail old conversations that contain important data that increases your search engine ranking for pages, but the request was how do I, not why shouldn't I...
Go ahead and open the "Member Management" link ("Members" in v7.6+) in the Control Panel and select Advanced and then select "before" and choose a date for the "Last Post Date" or "Last Online" (keep in mind, quite a few users have account on forums and don't post; myself included) and click search. On the results page you can select "Delete" from the "Actions" area (bottom of the results page) to prune those users shown.
