No, I think people can be persuaded on terminal values, although to an extent that modifies my response above; rationality will tell you that certain values are more likely to conflict, and noticing internal contradictions--pitting two vales against each other--is one way to convince someone to alter--or just adjust the relative worth of--their terminal values. Due to the complexity of social reality I don't think you are going to find too many with beliefs that are perfectly consistent; that is, any mainstream political affiliations is unlikely to be a shinning paragon of coherance and logical progression built upon core principles relative to its competitors. But demonstrate with examples if I'm wrong.
If you can persuade someone to alter (not merely ignore) a value they believe to have been terminal, that's good evidence that it wasn't a terminal value.
A long blog post explains why the author, a feminist, is not comfortable with the rationalist community despite thinking it is "super cool and interesting". It's directed specifically at Yvain, but it's probably general enough to be of some interest here.
http://apophemi.wordpress.com/2014/01/04/why-im-not-on-the-rationalist-masterlist/
I'm not sure if I can summarize this fairly but the main thrust seems to be that we are overly willing to entertain offensive/taboo/hurtful ideas and this drives off many types of people. Here's a quote:
The author perceives a link between LW type open discourse and danger to minority groups. I'm not sure whether that's true or not. Take race. Many LWers are willing to entertain ideas about the existence and possible importance of average group differences in psychological traits. So, maybe LWers are racists. But they're racists who continually obsess over optimizing their philanthropic contributions to African charities. So, maybe not racists in a dangerous way?
An overly rosy view, perhaps, and I don't want to deny the reality of the blogger's experience. Clearly, the person is intelligent and attracted to some aspects of LW discourse while turned off by other aspects.