I am also encountering a similar issue when attempting to deactivate the plug-in, this time on a custom theme. I receive the same error message:
Fatal error: Call to a member function delete_dismissed_notification() on a non-object in /nfs/c10/h07/mnt/177206/domains/cgt.columbia.edu/html/wp-content/plugins/constant-contact-forms/constant-contact-forms.php on line 318
