Counter act it with, "well why do you think that," or "what made you come to that opinion?" Those two questions always shut people up in debtes I've gotten in at school in my English or debate classes. They don't have a reason for it.
Or their answer is always "because that's what I think." At least give a decent answer saying something like "That's what I was taught growing up." (answers vary on the type of question of course

) It's on the lame end of answers, but it is still better then "just because."
May I quote something I've already quoted? Excellent! Here it is:
Caller: There's a question over that. I think a lot of people will tell you that. You're telling us one thing...
Paul Keating: Well, if you're sitting on the title of any block of land in NSW, you can bet an Aboriginal person at some stage was dispossessed of it.
Caller: You know that for sure, do you?
Paul Keating: Of course we know it for sure!
Caller: Yeah, [inaudible].
Paul Keating: You're challenging the High Court decision, are you? You're saying the High Court got this all wrong.
Caller: No, I'm not saying that at all! I wouldn't know who was on the High Court.
Paul Keating: Well, why don't you sign off, if you don't know anything about it and you're not interested. Good bye!
Caller: Yeah, well, that's your ...
Paul Keating: No, I mean, you can't challenge these things and then say, "I don't know about them".
John Laws: Oh well, he's gone.

EDIT: This may not be an example of tact, but it's certainly an example of someone not having a clue and trying to pull the opinion card (and definitely not getting away with it

).