On occasion you may want a blog category not to show up in your blog menu. Many themes now a days are including settings you can change to remove categories from menus but if you end up having to hard code it in here is what you need to do.
Open up the header.php file located inside the folder for your wordpress theme. You will need to replace the #3 in the code below with the id number of the category you wish to not include in your menu. If you categorize your entry in more then one category your category will still show up in the blog listing if your theme shows which category each entry is in. If you want to exclude more then one category simply add a comma after the first category number and enter the next category number with no spaces.
This will not remove the category from the frontpage. To remove the category from the frontpage check out my blog on Excluding a Category from your WordPress Frontpage.