The obvious answer is to flag them as Needs improvement. However this appears to be ineffective because all this does is push the items into the Close Votes queue and there do not seem to be any active Close Votes reviewers (0 reviews today). Every single close flag I have raised is either on a question that would have been closed anyway, or aged away.
As such, I turned to flagging as Very low quality instead because these flags will not age away and will go to the moderators who can unilaterally close questions. But they all get declined as:
Declined - This is what downvoting and comments are for.
How can I more effectively get an off-topic question closed? There are not enough Close Votes reviewers to actually make the close flags have any effect whatsoever. And the Very low quality flags get declined. And all the other flag options are not appropriate for off-topic questions.