The items on the list under the option Create Purchase Invoice are timesheets with a pay element which has not been processed. If you do not wish to process the timesheets and create the invoices then the process will be to remove the pay element of the timesheet.
Make a note of the timesheet number and period end and go to the Timesheet Display area, select the candidate and then the timesheet. From this screen you should be able to delete the pay rate line(s) from the timesheet - you may need to click Show Complex Rates in order to expand and separate the pay and bill lines:
Now you should be able to delete the pay rate line(s):
Click Update to save the changes. The system may display messages to state the pay and bill lines are not equal or that you have already invoiced, but you should be able to click Override and make the changes.