If you do not register your name or product. Can you stop a competitor from using it?
If you do not get around to registering the business or product name that you are using, you still have an unregistered trademark based on the reputation and goodwill that you have established. Although this sounds good, in practice it is very difficult and expensive to win an action against another who is using your unregistered trademark. They must be engaging in deception, passing themselves off as you or pretending that they are selling your products to customers who are confused. Even if this is just what they are doing, proving it may involve expensive market surveys to demonstrate the confusion and financial evidence that your business is being damaged.
Generally, registration of your name by another (unless it is a very famous brand) cannot stop you using the name if you can prove you have established reputation and goodwill in that name.
All in all, if you have a name worth protecting, it is best to register it.