For removing (changing) them from one skin, you would just explicitly override the rule properties that you want to change.
Or, you could put the rules you want to keep in your portal.css for each portal and empty the skin.css
If you wanted to do it programatically for one skin you could remove the link that references the default.css with code in the skin.ascx file something like this:
<%
Private Sub Page_Init(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Init
Dim oCSS As Control = Me.Page.FindControl("CSS")
If Not oCSS Is Nothing Then
Dim oLink As HtmlGenericControl
' find the default stylesheet link control
oLink = CType(oCSS.FindControl(CreateValidID(Common.Globals.HostPath) & "default.css"), HtmlGenericControl)
If Not oLink Is Nothing Then
oCSS.Controls.Remove(oLink)
End If
End If
End Sub
%>