In Edit mode, open the Individual Datasheet by clicking on the tab if necessary, and select the name of the individual to be deleted in the listbox on the right-hand side, if necessary typing a search string in the edit field above to find the name. Then click on the Delete button towards the bottom of the window.
Deleting an individual removes all references to the individual in the data records. You should not delete all the individuals who comprise a family, without first deleting the family itself. Deleting the individuals will not delete the family, but merely leave it without any members. You should also first ensure there are no references to the individual as an associate of another individual (as displayed on the Details page).
Before leaving View/Edit modes, you should also save the gedcom file to disk, by choosing Save from the File menu, or clicking Save on the toolbar. You will be prompted to do so.