In Microsoft Word, you can add or delete diagonal borders by selecting a cell in the table, and going to the "Borders" drop down menu in the "Design" tab.
To delete diagonal borders, select the cells that contain them, open the "Borders" drop down menu, and click on the diagonal border option that needs to be removed.
As far as I can know this does not work unless the selection of cells is continuous. Alternatively, you can select all cells in an area, add the same diagonal border and then remove it again.
Another option is to use the "Borders and Shading..." option at the bottom of the same "Borders" drop down menu. From here it is possible to customize any cells with more detail.