Select the cell or text you want to strike out
Press Ctrl+1 to open Format Cells
Go to the Font tab
Check Strikethrough
Click OK
To strike out only part of the text in a cell, double-click the cell or press F2, select the text, then apply Strikethrough
On Mac, press Command+1 to open Format Cells, then check Strikethrough
You can also use the Font dialog from the Home tab by clicking the small launcher arrow in the Font group
If needed, add Strikethrough to the Quick Access Toolbar or use a custom keyboard shortcut through macros