First of all, if this man, or any other man in a business should ever say or do anything inappropriate to your daughters, please, PLEASE go immediately to the management and make them aware of it!! I work in a pharmacy in a local grocery store and we have several of our "bag boys" who are mentally or emotionally challenged and sometimes needs to be "reminded" of the proper way to deal with customers!!!
That being said, I would say that you are already approaching this in a great way!! I would tell them that they are ultimately reponsible for the way they act and deport themselves and they can only learn to steer clear of the people ( both men AND women) who do not act reasonably and correctly! It also sounds like you have the type of relationship with your girls that would enable you to just have a frank, age appropriate discussion with them. Let them guide you, their questions and responses will tell you what questions they have and how they are accepting what you are telling them. And of course you know that this is not going to be a one shot discussion,it will continue over the next few years, and will grow and evolve as your girls do. The most important thing that I think I tried to impart to my (now grown) daughters was to be proud of themselves, and know that they had the RIGHT to expect...the right to DEMAND to be treated with respect and consideration!!! Do they have a good relationship with their Dad? That to me is one of the first steps in helping young women relate to young men in a healthy and postive way. I learned most of my expectations about relationships through discussions with my Dad!! He told me the way that the male half of the relationship looks at things and how they react to provocative dress or conversation.
I know I didn't give you very much concrete advice but I just wanted you to know that I think you are already on the right road!!! Congratulations!!