Please note: This is copied off the web and not my explanation... (don't want to get in trouble you know)
Using strikethrough font in blogger is overrated and entails an extremely convoluted process helpful and very easy to implement. It uses a simple html tag, which is just code that tells your editor how to display certain things. Html tags have to be opened and closed. Tags are opened by putting a greater than sign (<), then the tag content, followed by a lesser than sign (>). They are closed by doing the same process except with a backslash (/) before the tag content.
To strike through a word, make sure you are on the "edit html" tab and then surround the word you wish to strike through with an "s" tag.
Substituting the "s" in these examples will perform various functions. For example, "b" for bold; "i" for italics; etc. Strikethrough will work for posts, but not for comments in blogger.
Now that you know how to strike through words, your posts will look pretty much the same as before certainly look innovative and creative.