Removing a Taxonomy Field value from a List Item

If you’re trying to programmatically remove a taxonomy field value from a taxonomy field, you’ll quickly discover that setting the value to null will throw an error. It turns out you need to explicitly null out each of the values in the TaxonomyFieldValue complex type first, then update your list item’s taxonomy field with the new value, like this:

Guid fieldId = new Guid("{B756F341-D446-4ad7-B435-AA09F46340A3}");   
TaxonomyFieldValue tfv = listItem[fieldId] as TaxonomyFieldValue;
tfc.WssId = 0;
tfc.TermGuid = null;
tfc.Label = null;
TaxonomyField tf = page.Fields[fieldId] as TaxonomyField;
tf.SetFieldValue(listItem, tfv);
listItem.Update();