In order to cancel a scheduled retrospective form in a team, please make sure you are the Admin of that team.
If so, you must select from the top menu drop down the team which retrospective you would like to stop sending. Then go to the Feedback page.
At the bottom of that page you will find an 'archive' button, please click on it and the retrospective form will resume being sent to members of your team.
In order to bring the form back, go to the Overview page and click on the arrow icon next to the Team status and the retrospective form will start to be sent again to your team members.