KieServiesClient.completeConversation() 

will unset conversationId just from KieServiesClient instance. It won't delete conversationId from other clients instances (ProcessServicesClient,UserTaskServicesClient...)

Kie server client stores conversationId as class attribute. It means that it is possible to have more conversationIds for one KieServiesClient (one for every client implementing AbstractKieServicesClientImpl) and it is not possible to unset conversationId for most of the clients (just KieServiesClient has completeConversation method).