Currently (up to and include Crucible 4.8.x) an admin can delete all classifications but one.
(Administration > Crucible > Edit defect classifications >*Remove this classification)

There is a minimum occurrence of 1 in the schema, so even if they delete it upon saving the settings it will reappear. (Administration > Crucible > Edit defect classifications > Remove this classification)

<xsd:complexType name="fieldGroup">
        <xsd:sequence>
            <xsd:element name="field" type="cru:field" minOccurs="1" maxOccurs="unbounded"/>
        </xsd:sequence>
    </xsd:complexType>