According to the people behind Unddit and Reveddit, if a user wants archived content they authored removed, they should contact and request deletion from Pushshift.io, an external service they use to fetch archived Reddit content.
Unfortunately, there's no way to prevent your individual Reddit posts and comments from being archived. Archiving depends on the moderators of each subreddit. They can either choose to archive or unarchive all posts and not individual posts within the subreddit.