It may or may not work. Since it is a "free trial," personally I wouldn't waste my time or the post office's, I'd just discard the product and write a letter to the company (possibly even sending it registered or with a delivery confirmation, and of course keeping a copy of the letter) stating that you attempted to return the product twice and that you will not waste your time doing so again. Request in the letter that no further product of any kind be sent, and that if any further deliveries are received they will also be discarded.
When someone sends you a product you do not want, and haven't requested you're usually under no obligation to return it (you can use it or discard it), and certainly aren't under any obligation to return it multiple times. I'm sure there's even a legal way to MAKE them stop, but that might mean involving an attorney.
The reason I'd send the letter (with proof I had sent it) is because then the company cannot say that you ordered the product and just are refusing to pay for it.