If an ambassador no longer wants to or cannot complete the freebie mission you may decided to delete them from that mission. To do so, all you have to do is open the mission in question and find the ambassador (note you can use the search field here). Then simply click the bin icon on the right, click ‘delete’ on the popup to confirm and you’re all done :)

See how here:
https://www.useloom.com/share/ca4dee8bd82a4bcebd6930dc1717cbcb

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