See solution in other versions of Excel:
Question: How do I draw a line through a value in a cell in Microsoft Excel 2003/XP/2000/97? (This is also known as a strikethrough.)
Answer: Select the text that you wish to strikethrough. This can either be the entire cell or only a character in the cell. While your mouse is over the selected text, right-click and then select "Format Cells" from the popup menu.
When the Format Cells window appears, select the Font tab. Check the Strikethrough checkbox. Click on the OK button.
Now when you return to your spreadsheet, you should see a strike through your selected text.