A case can't be deleted if any financial matter postings including time have been done, even if they have been reversed off and put on the correct matter.
The advice would be to change the matter details and close the matter down.
The matter/case will need to be closed following the below steps:
Click on File then Click Matters.
In the Matter Ref box, put in the matter ref then click Search.
Click in the Details box and write DUPLICATE FILE - DO NOT USE.
Tick the box next to Close.
Click Save.