Resetting redux state on logout

You can make an action for clearing the store and use it in your rootReducer as like this:

const appReducer = combineReducers({
    ... // your reducers
});

const rootReducer = (state, action) => {
    if (action.type === 'CLEAR_STORE') return appReducer(undefined, action);
    return appReducer(state, action);
};

const composeEnhancers = window.__REDUX_DEVTOOLS_EXTENSION_COMPOSE__ || compose;
export const configureStore = (initialState, history) =>
    createStore(
        connectRouter(history)(rootReducer),
        initialState,
        composeEnhancers(applyMiddleware(routerMiddleware(history), thunkMiddleware))
    );

You'll have appReducer with all your reducers but also rootReducer which will be the function that will determinate returning a corresponding value or an undefined one.

TIP: You can use your Logout action rather than using a new action just for clearing the store

I wanted to point out to a couple of things that are pretty important before I give you a solution:

1. When using redux you should never attempt to alter the state of your store directly. The state should always change through the reducer as a reaction to an action. Therefore, reassigning the parameter initialState inside that subscribe callback is incorrect and pointless.

2. I don't think that you want to "reset" the state of the router property, correct?

One way to solve this is to use a reducer enhancer, something like this:

const resetEnhancer = rootReducer => (state, action) => {
  if (action.type !== 'RESET') return rootReducer(state, action);

  const newState = rootReducer(undefined, {});
  newState.router = state.router;
  return newState;
};

And then when you create your store do this:

  const store = createStore(
    resetEnhancer(createRootReducer(history)),
    initialState,
    composeEnhancers(applyMiddleware(...middleware), ...enhancers)
  );

And in that subscribe callback do this:

if (!session) {
  store.dispatch({type: 'RESET'});
}

One last extra-tip: since you are using redux-saga I strongly suggest that you move what you are doing inside that subscribe callback into a saga.