Remove filter of a plugin
I think you can achieve this goal, using one of the following solutions depending on the way this plugin instantiates the class:
if( class_exists('DM_Reviews' ) ){ //This should work in whatever case, not tested remove_filter('comment_form_defaults', array( 'DM_Reviews', 'reviews_form')); //or Instantiating a new instance, not tested remove_filter('comment_form_defaults', array( new DM_Reviews(), 'reviews_form')); //or Targeting the specific instance, not tested remove_filter('comment_form_defaults', array( DM_Reviews::get_instance(), 'reviews_form'));}Hope it helps, let me know if you get stuck.