I have experienced this issue as well. It appears that when you remove a member from a group, it leaves thier subscriptions untouched. I had to reassign my user to the group that had access to the board in question to be able to unsubscribe her from that forum. I thought that it was probally due to the instant subscriptions mod that I had installed, but know I think that it may be a bug.
Does anyone have an unhacked board that they can test this on?
I am currently running 6.3.1.
