Make a note of template, aspect , result set & result date.
Tools > Performance > Assessment > System Utilities > Results.
Delete a selection of results > Next
Select the required Students > Next
Select the Aspect > Next
Select the Result set (if there is one attached) > Next
Select the Date range of results to be deleted> Next
Select the Result entry date of results to be deleted (normally the same as above) > Next
Select the Templates which the results need to be deleted from > Next
Select any OMR sheets the results need to be removed from > Next
Select the any progress grids the results need to be removed from > Next
Finish
The results should now be removed from the marksheets