No refunds done even if they do not have what you need

My 15year old son bought a cat collar and it was too big, he bought this on his own with his pocket money. The lady that assisted him never mentioned anyhing about no refunds she said that if it doesn't fit he can exchange it. I went in with him when he wanted to exchange the collar for a smaller size. They could not assis us as there was no smaller sizes. When I told the assistan that in that case we would like to have a refund if they cant assist us, she then point to the "no refunds" sign. I was shocked. What is this??? Isn't it even legal not to allow refunds? And surely there must be an exception to this funny silly rule???? If the store can not assist with the right size who's problem is this??? Must the big collar just layin a drawer??? Our cat won't grow into this collar. I am disguisted and will never buy from this store again. I want the money my son spent back, and surely there must be a consumer law that can assist with this?? #vetmartmustfall