Legal Question in Business Law in California

If you own the fictitious business name and someone else is using that name in a way that is identical or similar to your DBA, you should be able to get them to stop. A cease and desist letter should go out to them informing them of your ownership in the name and demanding that they stop. If that does not work, then legal action may be necessary.

As a Franchise Attorney I would like the add the following. Assuming your DBA was also used as a trademark or service mark, you may also have a cause of action for infringement. Depending on how the mark was protected, this may be a stronger way to go. Consult with a good business or franchise attorney in your area for specific advice.

Mr. Franchise - Kevin B. Murphy, B.S., M.B.A., J.D.

Franchise Foundations, a Professional Corporation

I am a little less certain that what the landlord has done is sufficiently "illegal" that you're going to be able to oblige him to cease and desist under any California or federal law. Registering a fictitious business name with Sonoma County does not give trademark protection per se. This is not to say that you haven't established some defensible right to the name otherwise, as by using it in trade.

Indeed, neither does the landlord's forming of a corporation with that name give him any trademark rights.

An action for unfair business practices under Business & Professions Code section 17200 might be the best way to challenge the landlord's activities, which do seem unfair, especially if you had a going business and not just an idea.