It is possible to remove a map attachment from several instances of a DITA map using its khubId by defining the map attachment at the resource level as follows:
<?xml version='1.0' encoding='utf-8'?>
<controlFile>
<resources>
<resource>
<filePath>_pufferfish.ditamap</filePath>
<attachments>
<attachment>
<isAttached>false</isAttached>
<khubId>vQlcqgYveKfDv53M5zn82Q</khubId>
</attachment>
</attachments>
<instances>
<instance>
<filePath>Novice.ditaval</filePath>
</instance>
<instance>
<filePath>Expert.ditaval</filePath>
</instance>
</instances>
</resource>
</resources>
</controlFile>