What legal action can I take against someone spreading rumors about my business causing a loss of revenue?

To begin with, if the person is making untrue factual statements about your business which damage its reputation, he or she may be committing defamation; if so, you could sue him or her for both monetary compensation (e.g. lost revenue) and/or for a court order (an injunction) requiring him or her to cease doing this.

In addition, depending on exactly what is being said or done, this person may also be committing tortious (or wrongful) interference with contractual relationships (e.g. contracts, service agreements, etc. beween your business and its customers) or tortious interference with prospective (or future) economic advantage.

From what you write, you may have one or more legal claims or causes of action against this person. You should consult with an attorney about your options and recourse.