You need to flag it for moderation attention, and explain you think the question should be asked on Drupal Answers.
Keep in mind that:
- Drupal programming questions are also on-topic on Stack Overflow
- Questions about module/theme settings are tolerated on Stack Overflow
It is probable the moderators on Stack Overflow would migrate the questions on Drupal Answers if it is the OP that asks for the migration. You cannot force a user who sometimes asks a question about Drupal, but that normally asks questions about programming, to use two different sites.
Moderators on Stack Overflow are probably more incline to migrate the question if it is first closed on Stack Overflow. That is at least what happened to me, but I also say I am a Drupal Answers moderator when asking for a question to be migrated.
When you see the user is new and asks mostly questions about Drupal, you could suggest to ask those questions on Drupal Answers. Apart that, there is nothing else that can be done.