Hi Ulodesk,
Here's a macro you might find useful. It simply goes through all the Styles in the active document and deletes any user-defined ones. Any paragraph using a deleted paragraph Style will revert to the Normal Style and anything using a deleted character Style should revert to the underlying paragraph Style.
Code:
Sub CleanUpStyles()
Dim oSty As Style
For Each oSty In ActiveDocument.Styles
If oSty.BuiltIn = False Then oSty.Delete
Next
End Sub