Hey Thomas, to my knowledge reactivating users and then removing them is the only way. However, it sounds like something you may be able to do with an API call, which isn't much better than having an admin reactivating the users before removing them.
Just a side question. Do they see the deactivated users still in the member list of the social group?
Currently I don't see the deactivated users anymore in the list, where I'm pretty sure that this wasn't the case before. (Jive-N)
I have been looking into this as well recently but haven't found any other option then the activating/deactivating one.