Deleting student submissions
There may be times that you want to delete an individuals submission. For example, the student contacts you to let you know they submitted the wrong file or a student has started a test submission in error and you want to reset it. You can delete a submission using the following steps
Open the assignment in the Course Content area or go to the Gradebook to view the assignment.
Click on the Submissions tab.
Click on a student’s name to open the grading platform.
Click on the three dots near the grading pill and click Delete or Delete Attempt.
Click Delete again to confirm the message that you want to remove the submission and any grade associated with it.
TIP: Search for the student's name or use the Needs Grading filter to identify only submissions that are ungraded.

You can also delete student submissions by deleting the assignment (thereby deleting all related submissions and grades) or hiding the assignment (making the assignment and grade inaccessible).
Blackboard Help Resources
https://help.blackboard.com/Learn/Instructor/Ultra/Assignments/Create_and_Edit_Assignments