These options let you "review the reviewer," to improve your own experience with IFDB and help guide other members to the best reviews. They make up IFDB's community moderation system for game reviews.
Helpful (Yes/No): Vote Yes if you think the review does a good job of sharing the reviewer's opinion of the game - it's respectful, informative, well-written, insightful, or just generally helpful. Voting Yes doesn't mean that you agree with everything the reviewer said, or even with her overall opinion of the game. Reasons to vote No would include being off-topic, disrespectful, or overly spoilery.
Promote/demote this user: These affect how you (and only you) see this user's reviews throughout IFDB. If you Promote the user, you'll see her reviews listed first when viewing a game; if you Demote her, her reviews will move to the end of the list. (You can view and change your promotions and demotions via the user filter editor.)
Plonk this user: This is the nuclear option, for someone you find so annoying that you never again want to see anything they write. If you plonk a user, all of her reviews will be omitted when you view game pages throughout IFDB. This only affects you - it won't actually delete the user's reviews or hide them when other members view the same game pages. (You can change your mind and un-plonk someone later via the user filter editor.)
Flag spoilers: Mark this review as containing unhidden spoilers. A spoiler gives away a secret that you're meant to discover in the course of playing the game, spoiling the fun of finding it or figuring it out on your own. After you flag the review, IFDB will put a warning label on the review when other users view it. This affects all users, so use this command responsibly. Don't flag a review for spoilers that are already properly hidden with "click to see" warnings - only flag it if the spoilers are out in the open.