Just discouraging someone from saying it will either make them defensive, feel like you're saying they aren't entitled to their opinions (which yes, but they won't take it how you mean it unless you get very lucky), and so express their opinions even louder or, well. I dunno but it's unlikely to encourage more critical thinking.

I am lucky because in my professional and home life I am able to forbid that sort of lazy non-argument (well, in my presence at least, and dear me I am sounding a bit dictatorial. I suppose big fat hippies do end up enforcing their views; "you WILL be tolerant and thoughtful". Well this one does). Ultimately it's a meaningless statement. It's like saying "I am willfully ignorant", and willful ignorance is something I cannot stand.

However out in the world I feel there has to be an encouragement to critical thinking rather than a discouragement from entitlement, it needs to be a productive discussion and few people are up for that. In practise I either risk offending people by being 'patronising' or whatever or just leave it unless their opinion is based on an error of fact.