New Post: Update Taxonomy field and clear value

Hello, I'm trying to use SPServices to clear the value of a managed metadata field. I can change the value of the field to a new term, but trying to remove the value in the field and leave it blank doesn't work. The code I am using is

$().SPServices({
    operation: "UpdateListItems",
    async: false,
    listName: "Ticket",
    ID: "3157",
    valuepairs: [["LeadOfficeTaxHTField0", "-1"]],
    ........

This will work on some managed metadata fields but not others. What I've found is that if a field accepts multiple values this method will result in an empty field as expected. I have another managed metadata field that is indexed and set not to allow multiple values and this same process will leave the field unchanged.

Has anyone seen this before? Is there a better way to clear the value from a managed metadata field?