If you delete the user that posted the spam, it will also delete all of the blog entries.
I recently had to do something similar: http://community.elgg.org/pg/forum/topic/820536/spam-cleanup-metastrings-table/
Once it's all cleaned up take a look at implimenting some preventative measures.
http://community.elgg.org/pg/plugins/project/774755/developer/RayJ/spam-login-filter
http://community.elgg.org/pg/plugins/project/821368/developer/Beck24/spam-throttle-18x
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