nasty attitude to staff and to me as a customer

Today the 24th of march 2016 I had went to sparkport to purchase items. Upon being served by a cashier who I prefer being served by as a customer I get approached by two ladies informing me that I must stop going to a specific cashier and that its against the company policy. I'm a customer to sparkport for the last 8 years and I buy items everyday. Upon being approached I felt I was spoken to in a very harsh way infront of the rest of the customers waiting inline and infront the staff that work there. I'm a regular and almost the entire branch knows me because I work for standard bank accross the road. I was very rudely approached, I could of been approached in a more proffesional way by letting me purchase what I had and then call me aside and speak to me away. I find it very strange that a company like sparkport would have a policy stating a customer cannot use a cashier of his/her choice. I would like to receive a copy of the policy. From 2 ppl approaching me I started hearing other people talk about the incident and the so called supervisors also took of with the cashier infront of me and the customers waiting. I find this very disturbing considering the work place I work for.