Update
extra background I probably should have mentioned I'm in the UK and the organization I work for is not very hierarchical in nature.
I kind of knew the name of the client from stuff id heard, and associated them with "bad" stuff but before writing this and before i went to my manager I did lots of additional research to make sure it wasnt just hearsay. (I highly recommend this, as some of my opinions towards them did change)
what happened next I have regular 1 2 1s with my manager and I brought it up with him. I mentioned the un-comfort I had morally with the client, I mentioned some key points and also brought it back to the product we would actually be working on for them and how that would increase the sales of a product that is currently under investigation.
It was helpful to centre it around that and back up my ethical concern with real information (which id obviously learnt in researching the client before raising my objection).
Being honest and up front about how in effect I wanted to be pragmatic (e.g. not interrupt the work flow by flat out refusing to work with them) but that if I continued working with them, day in day out, it would probably weigh on me and lead to me exiting. I summarized this by saying "I wouldn't consider a job working for them directly, because of this"
That honest conversation then flowed naturally to a point where they wanted to see the information I had found, so I sent them all my research etc.
He then promised to have (and has since had) conversations with people involved in keeping / acquiring clients and will get back to me.
For me, knowing its being taken seriously is enough for now, perhaps my research is too biased and they will have a justified reason for continuing to work with them, perhaps they will plan to phase them out over time, perhaps it will go quiet for a bit and I will have to bring it up again later. Either way knowing it wasn't simply "their way or the highway" was really good and everything I could have hoped for at this stage :)
Thanks for all the answers, some really good discussion going on, I firmly believe if your working somewhere where decisions that matter to you are "above your paygrade" you should consider somewhere else because ultimately it will make you unhappy