Click on the timestamp of the post and then click the "flag" link. A few members in good standing flagging a bad comment will kill it. That's the most expedient

Click on the timestamp of the post and then click the "flag" link. A few members in good standing flagging a bad comment will kill it. That's the most expedient way to prevent a troll post from spawning a big off-topic thread that detracts from the content of the submission. It's also good practice to downvote a post like this before flagging it. They are separate actions, and once an account's cumulative karma drops to -12 (IIRC), all of its posts become dead-by-default.