In Civil 3D you can delete styles one at a time by right-clicking on them, or even purge large numbers of styles all at once with the Purge Styles command. But what about those styles that Civil 3D will not let you delete or purge? If they can not be deleted that is typically because the style is in use. It might be used be an object in the drawing or it might just be set as the default in a command. Either way, Civil 3D will consider it in use and not allow you to delete that style.
You can quickly tell if a style is in use by looking at the style on the Settings tab of the Toolspace. If there is an orange triangle beside the name of the style then it is in use. You will see that on several of the Surface Styles in the graphic to the right.
To find out where the style is being used right-click on the style and select Find References. This will display a dialog box listing all objects and settings where the style is in use.
To replace the style that is in use with another style right-click on the style and select Replace With. Then select the desired style that you want to use.
Now that the objects and settings are using a different style you can delete the original style in question.