We do delete spam threads, but some stuff still gets through.
Every SMF forum is currently having this problem, as it is being targeted by a robot spam attack. We have tried adding more human checks on account creation, but it isn't working very well.
Also, let me clarify that this isn't anybody's "job" as this is a site that is provided free of charge, even though it costs several hundred dollars a month to run (chat bandwidth is high). We are doing the best we can with volunteers who are all doing a good job.
Once SMF 2.0 gets out of beta, we will upgrade and this will stop the robot spam. We will also continue to look at ways to filter traffic better.