Strike through feature

Q: How do I achieve the effect of having your text look like it's been struck through?

A:  If you are using the updated post editor, there is a "strike through" icon on your tool bar (ABC)
First you highlight the text you wish to strike through. Then you click on the icon.
Publish post

source:
http://www.google.com/support/forum/p/blogger/thread?tid=4e155f459aebc9e0&hl=en