Once a result is saved against a student it is stored on the aspect and result set if one is used. This can only be removed by entering the history or in bulk via the below route so if backspacing the result no longer shows but returns upon accessing the marksheet again please see the below.
Right click on the result and select "View/ edit result history" click on the result you want to delete and a red cross will appear in this new window below the cross to close. Click on this to delete this result.
Alternatively you can delete a group in results in bulk by taking note of the aspect and result set name (if one is used) and going to Tools | Performance | Assessment | System Utilities | Results | Delete a selection of results.