Let me put something first: I'm running a "challenge" for the two guys in my group that actually enjoy optimizing at the moment. I informed them that it was gonna be hard, that we wouldn't spend too much time on character backgrounds and that I expected them to create optimized characters to act as infiltrators/spies/assassins of their church.
Now, they are both level 10, with LA +1 for free, a bonus feat and some extra cash to make up for the fact that they are only 2. For the "challenges" we'll always assume a 007-like situation, where they get told to infiltrate that evil church oder assassinate this guy, which involves a lot of mean traps, one or more difficult combat encounters and them covering their tracks afterwards.
If I now decide to challenge them with an especially difficult encounter (a hangman-golem, say, that may be easy or lethal depending on whether they discover the "vulnerable to fire/rope trick" thing or spot the golem in advance), even one that is in CR above what a party of four should be facing it's perfectly okay as long as I believe that they can actually beat it but have to really try in order to do so.
Why I am telling you this? That I'm doing this with these two players underlines what BruceLeeroy said, namely that the rest of our group just wouldn't enjoy it. If a group creates characters like this shinobi-guy they don't display that they are worse players than you and should thus be told the "error of their ways", but rather that they probably don't enjoy reading through a forum like this for hours and hours to learn new tricks to actually make the most of what you've been given. I certainly do, though, and so does part of my group - just not all of them. One is content playing a dwarf fighter over and over, and why shouldn't he? Clearly more work for the DM to make his role meaningful from time to time, but nothing you couldn't do with the help of the rest of your group.
If you decide that they certainly have to be taught that BFC is so much better than blasting and a straight Druid of their ECL probably beats the crap out of their groupsetup you better choose a different approach, because TPKing a group usually doesn't much help the players' fun. This doesn't mean you shouldn't surprise them with classes and combinations they don't use, but in order to challenge their innovation and not to show them that you win D&D and they don't. And if you see in advance that the group doesn't fit your expectations and they refuse to change the group, then it's up to you to adjust your expectations I guess and not to outright kill them.