How can I do synchronous test or not sharing a stub service?

I have a few tests that needs a service. I am using a stub service named UserServiceStub that I inject in my SettingsComponent, which I am testing. I reset the UserServiceStub in each test, but, tests are executed asynchronous so when I reset my UserServiceStub (which is an injectable and singleton class) other test has already initialized it. My tests are weak and asserts are wrong :-(

I would like to do synchronous test or a different UserServiceStub instance for each test, but I don't know how to do it...

I tried to define a method in UserServiceStub called reset() that resets stub service, but because it's asynchronous it doesn't work. Also, I executed in afterEach() method, the TestBed.resetTestEnvironment() method but it does not work either.

My reset() method:

reset() {
    this.user = { IBAN: '', SWIFT: '', paypal: '', app_idioma: '' };
  }

• The property I am using is called user and it has being overwritten by other tests

My beforeEach() method is this one

 let fixture: ComponentFixture<SettingsComponent>;
 let settingsComponent: SettingsComponent;
 beforeEach(() => {
    TestBed.configureTestingModule({
      declarations: [SettingsComponent],
      imports: [],
      providers: [
        TranslateService,
        AppService,
        { provide: MockBackend, useClass: MockBackend },
        { provide: HttpToolsService, useValue: HttpToolsServiceStub },
        { provide: BaseRequestOptions, useClass: BaseRequestOptions },
        { provide: UserService, useClass: UserServiceStub } // My stub UserService
      ]
    });

    fixture = TestBed.createComponent(SettingsComponent);
    settingsComponent = fixture.componentInstance;

  });

A test:

 it('#updateSettings should  doesn\'t update the user\'s settings when is wrong', fakeAsync(
    inject([UserService, HttpToolsService, MockBackend],
      (userService: UserServiceStub, httpToolsService: HttpToolsService, mockBackend: MockBackend) => {
        userService.reset(); // It does not work...
        userService.isError = true;
// However, I have reseted my user in UserServiceStub,
// this returns false because settingsComponent.getSettings().IBAN is actually 'XXX' 
// instead of '' (empty) how it has to be
        expect(settingsComponent.getSettings().IBAN).toBe(''); 
      }))
  );

I hope your answers, if you got any question just ask.

Thanks :-)