After moving this site to a host with cron access I cleaned up a remaining 165k spam registrations with a scheduled task run one minute before and the following job. 175 users was the most the server could delete at one time but your millage may vary.
9,19,29,39,49,59 * * * * /usr/bin/mysql -u root -p<MYSQLPASSWORD> -e "USE <DBNAME>;UPDATE user set usergroupid = <DELETIONGROUP> where usergroupid = <SPAMGROUP> LIMIT 175" > /dev/null 2>&1
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