Steps:
1) click on View responses to get the Responses of the users
2) select one or more users and choose "Delete" from the Drop down menu "Choose an action..."

I get the following message:
Notice: Undefined variable: DB in /var/www/moodle25/mod/choicegroup/lib.php on line 591

Fatal error: Call to a member function get_record() on a non-object in /var/www/moodle25/mod/choicegroup/lib.php on line 591

I fixed the error adding
global $DB;
in line 575 file lib.php