I think you've created a pretty serious problem for yourself.  In my opinion, in a situation like that, you should have shut down the forum until repairs were made.

I haven't used the Access version in several years, but I assume the ID field is auto numbering, so you can just copy and paste from the backup MDB into the production MDB.  The auto-numbered Thread_ID will set new ID numbers for the records being pasted in.  You should keep a record of the ID's that are created during the combining of the data.  If only the tblThreads was emptied, then the links to at least Topic, Forum, and Author should be intact.  However, since you said the users and polls are having the same problem, are we to assume that every last table was blanked out?  If all the tables have been rebuilt from ID zero then there is no way you can do a simple import.  It's not impossible, though it will require a lot of manual adjustments.  Someone with an in-depth understanding of how WWF works and Access expertise will have to do the work.

In any case, if you suspect that someone acquired your Admin password, you need to figure out how to prevent that.  Using the web interface to perform a brute force attack against a 160-bit encrypted password would be highly impractical.  Such an attack would be far more plausable (though still not a trivial task) if the attacker was in possession of the hash and salt, but even then a hacker would be wasting his time to try break the password since there are easier ways to acquire admin rights in WWF once in control of the raw data.  You have to take every precaution and use every "best practice" to guard the MDB.  Make sure your host (FastHosts?) has configured the location of the MDB file so it is outside and inaccessible from the wwwroot.  It should not be in a virtual folder.  Make sure you don't have any domain user accounts with access to the folder wherein the MDB is stored.