application_controller.rb

class ApplicationController...
  def clear_cart
    @cart.destroy
    session[:cart_id] = nil
  end
end

cart_controller.rb

class CartController < ApplicationController
  def destroy
    @cart = current_cart
    clear_cart
    respond_to do |format|
      format.html { redirect_to root_url, notice: 'Your cart is currently empry' }
      format.json { head :no_content }
    end
  end
end

line_items_controller.rb

class LineItemsController < ApplicationController

  # DELETE /line_items/1
  # DELETE /line_items/1.json
  def destroy
    @cart = current_cart
    @line_item = @cart.remove_product(params[:id])
    
    respond_to do |format|
      if @line_item.save
        @line_item.destroy if @line_item.quantity == 0
        if @cart.line_items.count.zero?
          clear_cart
          redirect_to root_url and return
        end
        
        format.html { redirect_to @cart, notice: 'Line item was successfully removed' }
        format.json { head :no_content }
      else
        format.html { render action: "new" }
        format.json { render json: @line_item.errors, status: :unprocessable_entity }
      end      
    end
  end
end