Simple answer, tell the guy to shove his idea about taking fat burners up his hole, with the non-existant commission payment he now won't get, for unsuccessfully trying to sell you such biologically loathesome filth in the first place.
Gyms having those things on the premises for sale is questionable at least, but someone recommending them to your face, should be worthy of a verbal warning I think.
My Gym sells some supplements, but never sells anything to me; at best a mere suggestion once in a blue moon, but nothing more, with only anything resembling sales patter when the person is queried about a product, or the general topic of converstion strays onto supplements.
If anyone in my Gym started hawking to me, I'd complain to the management, but as their products are profit orientated only, with no commission structure, there's no incentive for staff to flog or hawk products and to be honest no Gym anywhere in the known universe period, should hawk supplements or equipment of any kind if they choose to sell things, but merely let hte individual enquire if they wish. Commission incentives should be for things like furniture, gas supplies, home insurance, cars etc, not supplements and workout equipment, except in an actual retail or warehouse type premises, not Gyms.