FCRemoveTypesettingRules[expr] removes all typesetting
rules attached to expr. Effectively it sets the
FormatValues of expr to an empty list.
Overview, FCAttachTypesettingRule.
ST1\text{ST1}
FCAttachTypesettingRule[ST1, {SubscriptBox, "S", "T,1"}]ST1S_{T,1}
FCRemoveTypesettingRules[ST1]
ST1\text{ST1}