The florist is either ignorant or dishonest. You cannot successfully grow a plant in such a container. Plants need to be drenched thoroughly on watering day in order that the soil/root mass can be uniformly moistened. Excess water will exit from the drainage holes.

Without the holes, no amount of gravel can save a plant from the rising water level and subsequent root rot.

Can you take the plant back?

I agree. Some limited number of people have been able too keep plants alive very short term without drainage holes, but it's really nearly impossible long-term to succeed without holes. Rocks at the bottom do nothing without a drainage hole. Everyone in the houseplant industry, from wholesale growers, to retailers, to authors of how-to books, to all the pro's who give advice on the plant forums... ALL of them know you must have drainage holes. Your florist has no business selling plants. You definitely deserve a refund if you want one.

No-drainage-hole pots can be used as decorative cachepots, or pots outside of a pot that does have drainage holes. There is a thread going on about them right now here in the Houseplants Forum.