If you know the ID of the Defect, it is easy to remove it from the database from Site Admin.

0) ensure you have a good, recent backup of the project DB and Repository

1) In Site Admin, select the Project

2) expand the project (plus sign at left of the name) to see the tables

3) Click on the ''BUG'' table

4) in the box just above the results table listing all bugs, you should see this SQL statement: SELECT * FROM BUG

5) Replace that with this SQL code (I am using ID=31 for an example, replace with your ID)
SELECT * FROM BUG WHERE BG_BUG_ID=31

That should select the single record so you can confirm that it is the right one.

6) Change the SQL statement to this:

DELETE FROM BUG WHERE BG_BUG_ID=31

[it should return ''empty table''

7) Try

SELECT * FROM BUG WHERE BG_BUG_ID=31

or

SELECT * FROM BUG

to confirm record is gone.

8) Users will need to get out and back into project or refresh Defects module to see change.