Certainly. Trademark registration is not required for trademark enforcement, it merely is a sure way of proving the establishment of your mark (trade or servicemark) and may entitle you to greater damages. But by using the mark in commerce you establish a mark whether it is registered or not. The strength of the trademark will depend on the type of word you are talking about. Fanciful marks are coined terms specifically created to function as a mark. think of Polaroid, Canon and Xerox. These words were unknown to consumers before being created as marks. They are the strongest marks and are very protectable, meaning you probably can't use the word even if you are using it for a different product like "Polaroid T-Shirts" of "Exxon Fertilizer."Suggestive marks suggest qualities which are desirable for a product, but do not actually describe them. Think of "7-Eleven" for a convenience store, "Seventeen" for a teen magazine, or "Best Buy" for cheap electronics. Though not as strong as fanciful or arbitrary marks, suggestive marks are distinctive and highly protectable. Descriptive, surnames, and geographical marks which are merely descriptive of goods or services, are much weaker than fanciful, arbitrary or suggestive marks. To be protectable, these marks must have secondary meaning. That means that out in the marketplace when people hear that mark, they don't just think of the generic word, they think of the product. It is this "secondary meaning" that generates the most litigation. Our firm has represented many businesses, large and small, that have sought to enforce or defend against these types of lawsuits. From what you are telling us it sounds like you stand a great chance of success. Also, a declaratory judgment is almost always part of a trademark lawsuit. It is a court judgment that declares a company's right to use a particular mark; so it would be very valuable to you and your company.
Finally, you may have other remedies for "going after your customer base" if he is tortiously interfering with your contracts with your customers for example. You should take a stand and do so quickly to avoid confusion and loss of business in the marketplace. Good Luck.
As Oscar notes, you need not do anything other than use a trademark in commerce to acquire trademark rights. Registration of those rights with your state and/or the federal government provides many benefits but is not required. In short, trademark rights arise naturally as long as the word, design, or phrase that you "use in commerce" as a trademark is not the very name of the product being sold or describes the product or a key element of the product. A trademark attorney will be able to determine if you have developed trademark rights in the mark being used to brand your product.
Two key points:
(1) Trademark rights are geographical in scope. For example, it is lawful for one person to use Trademark X to sell certain goods in Florida while another uses the identical trademark to sell those same goods in Alaska. While the internet advertising of products has complicated the question of the scope of trademark rights, this principle is still firmly applicable. In short, determining the scope of trademark rights is sometimes a complicated question -- but which must be answered before concluding whether someone has "infringed" your rights.
(2) Trademark rights arise only if the mark is used in commerce. According to trademark law: "The term 'use in commerce' means the bona fide use of a mark in the ordinary course of trade, and not made merely to reserve a right in a mark." The rule is that the occasional sale of a product -- either in the course of a business or "as a hobby" -- does not rise to the level of the "ordinary course of trade" and so is insufficient to create trademark rights in the mark that is used on the product. Without more information about the extent of your use of the mark, no one can tell you if you have developed rights in the mark you have been using.
General information about trademarks and the registration process is available at from the US Patent and Trademark Office at http://www.uspto.gov/web/offices/tac/tmfaq.htm
Just an additional note in addition to the fine answers given above - my colleague, Jim Harris, wrote an exceptional article on the difference between trade names and trademarks. You can read it at the link below.