A [question that is migrated to another site is rejected][1] by that site if:

- it gets deleted
- it's closed as Not Constructive, NARQ, Too Localized, or generic Off-Topic

To reject a question that's been migrated here, you should vote to close for one of the reasons listed above.

When a question is rejected:

 - it's unlocked (but not reopened) on the source site
 - any answers that were deleted as part of the migration are undeleted on the source site
 - all migrated answers are deleted on the destination site
 - the migrated question and any answers received on the destination site are locked on the destination site

I can't find a definitive reference, but there are a few mentions of rejected migrations being deleted on the site they had been migrated to.  (After 30 days?)

  [1]: http://meta.stackexchange.com/questions/35744/allow-diamond-moderators-to-reverse-question-migrations/126982#126982