Then encourage them to get feedback from family and friends. Chances are if your assessment is correct then your client will get the feedback not only from you but from others whom opinions they respect. In the end this is about all you can do.
Having clients means often having to do your best within the parameters that they set for you. They are the ones paying and you are the one working after all.
The other thing that is very concerning in your post is that you think this one issue is so bad that you don’t want to be associated with it. The truth is that this is going to happen over and over again in your career and you are going to need to learn to make the best of it.
Nothing says you can’t be associated with a project and also discuss, on your resume or web site, how if you were allowed to make the decisions you would have done things differently. I think if you take this approach you can gain credibility with future customers so that it might make it easier for your opinion to count more.
Distancing yourself from too many projects in the long run will make you appear inexperienced and eventually may damage your reputation much more than being associated with a project you are not completely happy with.
Best of luck to you.