domain mapping error in debug log

These log messages are filling up my debug log. Is there a fix?

[21-Jun-2013 22:44:34 UTC] PHP Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at /home/wordpres/public_html/wp-content/mu-plugins/domain_mapping.php:491) in /home/wordpres/public_html/wp-content/plugins/stop-spammer-registrations-plugin/stop-spammer-registrations.php on line 72
[21-Jun-2013 22:44:34 UTC] PHP Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/wordpres/public_html/wp-content/mu-plugins/domain_mapping.php:491) in /home/wordpres/public_html/wp-content/plugins/stop-spammer-registrations-plugin/stop-spammer-registrations.php on line 72
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