Best thing to do is probably recreate the deleted node/s and forum/s, in the database change the node IDs accordingly. Then delete the nodes again. This should delete everything cleanly, including the posts and any other associated data.
I must stress, though, it's not exactly "officially" recommended to do that. You should probably test it in a test environment first. Or, at least, take a back up before doing anything, and be prepared to restore that in case it doesn't go as expected.