1. Navigate to the ServiceDescripttion.xml file of this service. This would be generated by the BizTalk wizard. You can go to C:\Inetpub\wwwroot\[servicename]\App_Data\
2. Find the xml tag Operation and replace the actual value for name attribute with an empty string.
3. Recycle the app pool
4. Browse the WSDL and it should not have the soapAction value