Download backups of your board, weekly, just to be safe, our backups run daily but if it backed up a hacked
board (meaning someone broke into an admin password by getting the information off your computer somehow or just guessing the username's password...) then our backup will not be usable to restore from. I would definitely download copies of backups, delete the old one and download a new one if you're really worried and you can feel better about things knowing you yourself HAVE a current usable backup in case our backup cycle runs after a password cracking of a user account. On our end things are as secure as you choose to make them, you can put the board in Maintenence mode and member by approval only or you can leave it as is with it's current settings.
Whatever makes you and your members feel better. If you and all admins enalbe the log me in only once feature you will be the only one able to log into your account. We have given boards and users the tools to secure their accounts and communities, so it's best to decide what you are willing to do in order to secure your own interests. I doubt they can get a member list off a board with it being disabled, the most they could see if the feature is enabled, is the last users seen on the info bar on the lower left and the In Power list if that too is enabled.
Let us know if we can help at all. Best of luck weathering this problem, I am sure you and your members will not be targetted for long.
If you need anything from us just let us know.