After changing the task owner remove permission for old task owner in approval workflow.

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C#

SharePoint2013

I am programmatically delegating task to another user for a Nintex approval workflow. For example actual approver is "DOMAIN\Test1". And the delegated user is "DOMAIN\Test2"; Using the below code its delegating the task to "DOMAIN\Test2" user but the problem is still the actual approver "DOMAIN\Test1" retain the permission to complete the task, I mean "DOMAIN\Test1" still is able to click the "Complete Task" button, actually this button should be hidden for him("DOMAIN\Test1"). How can I do this?

using (SPSite currentSite = new SPSite("http://tst:001/sites/Test"))
{
  using (SPWeb currentWeb = currentSite.OpenWeb())
  {

      string tasksList = "Workflow Tasks";                    
      SPUser delegateTouser = currentWeb.EnsureUser("DOMAIN\\Test2");
      SPListItem listItem = currentWeb.Lists.TryGetList(tasksList).GetItemById(101);
      listItem[SPBuiltInFieldId.AssignedTo] = delegateTouser;
      listItem[SPBuiltInFieldId.WorkflowVersion] = "1";
      listItem.Update();
  }
}