Can't moderate 100,000 posts - crashes forum!

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Please Help - Spammers have crashed my forum!

I have quite a large forum, with over 15,000 pages indexed in Google. Russian spammers have started targeting my forums with spam bots, registering dozens of new members at a time and firing off posts to every thread simultaneously.

My moderators all quit because they couldn't handle the moderation, and unfortunately I was slow in realising the scale of the problem and reacting. Now my forum has crashed!

The "vb_post" table is about 1.5 Gb and so unstable it corrupts every time I run an operation.

I have over 100,000 unmoderated posts. I want to bulk delete them, but as soon as I click on the "Moderate Posts" tab in the Admin CP it crashes and corrupts the table.

Trying to Repair or Optimize the "vb_post" table under Maintainance also crashes and corrupts the table.

My hosts can repair the table for me by running a mysqlcheck with the -r option from their end, but they have already done this for me 5 times, and are running out of patients. They have told me to reduce the size of my database or they will have to delete it, but every time I try to prune posts it corrupts the table again!

Can anyone help or offer any advice?

If I could only delete all unmoderated posts, and all posts by deleted users, it would free up about half my post table, but I don't know how to do that without it crashing!

Can I manually delete all unmoderated post by running a mySQL command from command line?

Perhaps I could stagger it, by limitting it to deleting 500 records at a time, so as not to overload the server?

Does this have knock-on effects on other tables, (vb_posthash , vb_postindex, wfvb_thread ???). Would I make things worse by maually purging this table?

What do other people do when their post tables reach 1 GB in size?

Please help me, this forum is over 7 years old, and I really don't want to lose everything!

Neil