Попробуйте, у меня работает.
public static class ControlUtility
{
public static void AddCssClass(WebControl control, string cssClass)
{
control.CssClass += " " + cssClass;
}
public static void RemoveCssClass(WebControl control, string cssClass)
{
control.CssClass = control.CssClass.Replace(" " + cssClass, "");
}
}
ControlUtility.RemoveCssClass(lnkletter, "ln-enabled");
ControlUtility.RemoveCssClass(lnkletter, "ln-disabled");