The thing is, "not an answer" and "a very bad answer" can often be really hard to differentiate between.
The better approach with that kind of answer is to flag it for being very low quality. It will then go into the low quality review queue for a while, and unless the community saves it by reviewing it as "Looks OK" enough times to get it removed from the queue, it will become a flag for moderator attention at some point.
The low quality queue approach also has the advantage of letting reviewers leave automatic comments on the answer explaining why it sucks, as well as exposing the answer to more potential downvotes, which contribute to the automatic question and answer blocks Stack Exchange has implemented. so next time, if there's any doubt at all, flag it as low quality.