You can delete a time entry from the Time Card screen by deleting the hours entered on the grid, by clearing them one by one. You must leave the box completely blank and the time entry will be deleted.
You can also select rows and then use the Actions > Remove selected rows option to delete time entries, provided they have not been billed. You can also remove all the blank rows from the grid by clicking Actions > Remove all blank rows. Check CORE Help for details.