Is there any one word to describe people who don't prefer the product?

Non-preferred or unpreferred are both correct but don't sound very elegant. Unfavored may suit your needs better.

People who don't have a preference could be considered indifferent:

[Merriam-Webster]
1 : marked by impartiality : UNBIASED
2 a : that does not matter one way or the other
2 b : of no importance or value one way or the other
3 a : marked by no special liking for or dislike of something
// indifferent about which task he was given
3 b : marked by a lack of interest, enthusiasm, or concern for something : APATHETIC
// indifferent to suffering and poverty

But, in terms of marketing, I'm sure that no company would like to think that somebody could never be swayed. So, from their perspective, people would never be indifferent, they would simply be undecided:

1 : not having made a decision
// undecided voters

Note that this answers the title of your question. The body of your question seems to be asking something else—not for the name of people who don't prefer something, but for the name of a thing that nobody prefers.

The name for a such thing would be unwanted:

: not wanted or needed
// attracting unwanted attention
// giving away some unwanted possessions

But, again in terms of marketing, companies might not want to think that any of their products are unwanted. They may simply choose to look at such products as nonprofitable:

: not profitable

profitable
: affording profits : yielding advantageous returns or results