How to disable a Org User?

One would have thought it would be easy to disable a user:

users = System.getModule("com.vmware.library.vCloud.operation.admin").getUsersAdminOrg(org);

System.log("User count ="+users.length);

for ( var user in users ) {
    user.isEnabled = false;
    actionResult = System.getModule("com.vmware.library.vCloud.operation.admin").updateUser(user) ;
}

What I get here is:

[2012-11-14 10:47:40.838] [I] User count =2
[2012-11-14 10:47:40.851] [I] TypeError: Cannot find function update in object 0. (Dynamic Script Module name : updateUser#1)

What am I doing wrong here?