If a data import doesn't go quite as expected, you can easily Undo the import.
Undoing the import will remove all records that the import brought into MyCase.
Open the Settings page.
In the left-hand menu, click Import/Export.
Then, click the section that contains the import you'd like to undo (Contacts & Companies, Court Cases / Matters, etc.)
You will be taken to a table that lists every import you have done in MyCase. On the far right side of each row, you will see an icon depicting a circular arrow. Mouse over this icon, and it will say Undo Import (pictured below). Click on Undo Import.
You will be asked to confirm the Undo of the import. Click Proceed to continue with the Import Undo.
MyCase will remove all of the records that were associated with the import.