Thanks for posting and welcome to the Bose Community! I would love to answer any questions you may have about your headset.
It will continue to charge as the iMac is still receiving power when off, this is normal! As far as I am aware this isn't something that can be turned off unless you disconnect the iMac from power.
Please do let us know if u have any further questions.
Feel more. Do more. Be more. Come check out my profile here. New to the community? Check out the Getting Started board. Help others find answers to your questions by clicking on 'Accept as Solution' at the bottom of a reply. If you see a post you like, make sure to give it a thumbs up!