The spammers have found you.
That's what happened to me this morning when I was checking on the blogs of two clients that we manage - one had over 10k, the other over 3,000 spurious, generic or plain spammy communications.
Close Comments on WordPress
The first thing you need to do is to try to stop the problem in the first place.
Go into Settings in the dashboard in the left hand sidebar. Then select Discussion.
Now you will have to uncheck the first three boxes which allow notifications to blogs/websites you are linking to, accepts trackbacks or link backs and allows comments on new posts.
Then check all the other boxes and save. This should stop comments on posts over 14 days old and make it so that anyone who does comment has to register an account with your blog first.
This should stop the spammers from being able to post on new or old blog posts.
However, some of them seem to have found a way to post comments on the pages that are created in some themes when you upload an image - that is a lot more difficult to prevent as it involves playing around with the code.
Delete Pending Comments
So, what can you do with all those pending comments that are sat in a queue waiting for you to moderate them?
Well, I found a great plugin called 'Delete Pending Comments'.
Click on Plugins and then Add a Plugin. Now search 'Delete Pending Comments' - it should be the first one in the list with about 25 happy reviews.
Add this plugin in the usual way and it will appear as an option in the Comments section of the dashboard. Select it and then type the confirmation sentence as instructed - you can't copy and paste, you have to type it out.
Click Delete Pending Comments and, Bob's Your Uncle, the comments all disappear.
I did have one site where I think it may have taken some of the Approved comments as well, but I couldn't get it to replicate elsewhere so I may be mistaken (having a senior moment and there were no Approved comments on there to start with). However, I think I should mention this as a possibility.
With luck, the measures you now have in place in your Discussions settings will stop the majority of the spam comment abuse so it doesn't reach those levels again but, every so often, you can go in and have a cull.
I deactivated my plugin on some blogs and deleted it altogether on others as I prefer to have as few plugins as possible taking up room on my sites.