How to remove SOAP header 'Security'

I'm trying to consume a SOAP 1.2 web service using a WCF client. The problem is, whenever I make a request a MessageSecurityException exception occurs with the following internal message:

SOAP header Security was not understood

As I understand it, WS does not understand the "Security" header, which is currently marked as MustUnderstand="true" in the SOAP envelope.

How can I change MustUnderstand to false / remove the "Security" header?

Below is the binding I'm using now:

<customBinding>
  <binding name="CteRecepcaoSoap12">
    <textMessageEncoding messageVersion="Soap12" />
    <security authenticationMode="CertificateOverTransport"/>
    <httpsTransport requireClientCertificate="true"/>
  </binding>
</customBinding>