Autodesk 24108-051400-9000 - AutoCAD Revit Architecture Suite 2008 User Manual page 539

If you want to
purge all unused styles or definitions of a selected type
If a style or definition you selected to purge remains in the list after purging, the style or definition is
currently applied to an object or is used in another style. For example, you cannot purge a wall endcap
style that is used in a wall style or that is applied to an individual wall.
4 Click OK.
Then
right-click the style or definition type, and click Purge. When
prompted, verify that the styles you want to purge are
selected, and click OK.
Purging Styles | 499