[Box logoutWithCallbacks:^(id <BoxOperationCallbacks> on)
 {
     on.after(^(BoxCallbackResponse response)
              {
              });
 }];

BoxSDK *sdk = ... // a reference to the BoxSDK for the user
sdk.OAuth2Session.accessToken = @"INVALID_TOKEN";
sdk.OAuth2Session.refreshToken = @"INVALID_TOKEN";

[BoxSDK sharedSDK].OAuth2Session.accessToken = nil;
[BoxSDK sharedSDK].OAuth2Session.refreshToken = nil;