post indexer network wide tagging is killing my site

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On a large multi-db based network of 30k blogs, we’ve had a LOT of spam signups recently for all the “black friday / cyber monday” spam blogs. Unlike previous spam blogs, these make HEAVY use of tags and categories. When a new spam blog is created, the owner/bot automates the process of posting a bunch of spam posts, each indexed by the post indexer plugin. Surely this wouldn’t be a big deal, except that in less than one day this balloons the size of the wp_site_terms, wp_site_term_relationships and wp_term_counts tables (40k+ per day), and since these records are attempted to be purged when the system deletes the blog, it kills my site each time I try to delete these blogs. Instead, I must first disable post indexer and purge these tables.

truncate wp_site_terms;
truncate wp_site_term_relationships;
truncate wp_term_counts;

Is there any way to reduce the load from this so it doesn’t keep killing the server? For now I have post indexer disabled.