I just did a quick test and these queries work for deleting all personal conversations. You can paste these queries into phpmyadmin:
Code:
TRUNCATE TABLE xf_conversation_master;
TRUNCATE TABLE xf_conversation_message;
TRUNCATE TABLE xf_conversation_recipient;
TRUNCATE TABLE xf_conversation_user;
UPDATE xf_user SET conversations_unread = 0;
Backup first just in case. My test was successful but I might have missed something.