This question already has an answer here:
Now that migration to other sites is quite limited, not being able to migrate questions which have been on the site for at least 60 days, it makes sense to apply those same rules to rejecting migrations.
Questions which have been migrated and sat around on the target site for months, a year, or two years should not be kicked back to the original site if they eventually get closed. The same rules of migration should apply here. If the question has existed on the target site for more than 60 days, closing it should just close it and leave it alone, leaving it unlocked and able to be reopened, edited, deleted, or whatever else the community wants to do with it.
Basically, after 60 days, the migration becomes permanent and can no longer be rejected.