Even if you don’t register your trade mark, you still have the possibility of stopping someone from using a trademark that is similar to yours on their goods and services. In this case you will generally need to get legal advice from a specialist e.g. trademark attorney.
This situation is known as “passing off” and in this case the trade mark is harder to defend than it is with a registered trade mark. To succeed in defending an unregistered trade mark you need to demonstrate that the trade mark belongs to you, you have built up goodwill associated with your trade mark and also, that by the other person using the trade mark your business has been harmed.