HELP! Can't get into admin after receiving this warning: Warning: Cannot modif

I received this warning after trying to activate a demigraphics plug in for google. I thought I deactivated it, but now I can’t get into either website admin. I just get this:

Warning: Cannot modify header information – headers already sent by (output started at /home/drolfes/public_html/sweetfatlies.com/wp-content/plugins/demographic-data-in-google-analytics/DGA.php:1) in /home/drolfes/public_html/sweetfatlies.com/wp-includes/pluggable.php on line 934