Here https://stackoverflow.com/a/13374264/6250381 is written "then update your schema", but how to make that?
Try using
/**
* @ORM\ManyToOne(targetEntity="Report", inversedBy="responses")
* @ORM\JoinColumn(name="reportId", referencedColumnName="id", onDelete="CASCADE")
*/
protected $report;
And then update your schema. It will add database level Cascading