Comment Probation allows you to require moderation for a future comment by a comment author, rather than having the comment be automatically approved.
Comment Probation allows you to put a comment author on "probation."
If you allow comments to automatically appear from authors who already have an approved comment, this plugin gives you a bit more control. It lets you approve a comment but place the author under probation, meaning that any future comments from that author will need to be moderated again.
When approving a comment, you're given the option to "Approve" the comment, or "Approve with Probation." If a comment ended up in moderation because the author is on probation, "approving" the comment will remove the author from the probation list.