If you are admin you can easly find item named unequip, and you need to chaose right one, for example unequip beard and give it to thrall, after that he will have item back in his inventory.
I answered this for other people seeking answers ;)

I am not an admin. How to remove accessories from Thralls ?

There is no gem to remove glasses from a thrall. I have tried unequip all and that does not work.

I tried all of the other gems also. :)