I think I'm actually a little confused what the problem is and why you'd flag them... People play differently in this game and are more willing to risk certain things. Sure, some people only go further when they have a 6 7 8. Some people see that as too safe, because if they actually make the weird column then they have a huge leg up on winning.
How many times do they fail when they do this? Them running up a weird column and failing a few times before they manage it may actually get them the equivalent of how far you get in 3 turns.
Also, when you see this, does it happen when you're close to winning? Because I'll tell you, if it's the choice for me between definitely losing next turn and seeing how far I can run up a weird column, I am absolutely going to try to run up the weird column to win. I've done that twice on an 11 before. Shocked the life out of me, but it worked. Sometimes you just have to work with what you have.
And in the end, sure, this is kinda a game of strategy. But it's also a game of luck. I didn't actually take the time to go an accurate calculation of the probabilities involved in that happening, but I seriously doubt it's all that low since the rolls aren't correlated. Some people just play the odds differently. That's where the strategy part of this game comes in.
What I do think is that it's kinda rude to flag people who use a different strategy than you (albeit a risky one) just because you think that means they're cheating... I think it's more fair to give people the benefit of the doubt. Actually, I'd suggest that if you haven't, go play some of the Can't Stop tournaments. The range of strategies is fascinating.