I need some advice on the following:

Our retail shop is located in a shopping centre. In October last year it came to my attention that the lady working at the optometrist across from our shop was selling cleaning chemicals similar to what we are selling. We reported the matter to the Centre's managing agency and they sent them a letter saying that she is in breach of her lease agreement.

She has now stopped selling from her shop, but one of my customers came to see me last week and told me that the same lady called her over after she left my shop and told her not to buy from me as she is selling chemicals cheaper from her house.

Now what do I do? Can I send her a Cease and Desist letter?