Here's what I'm doing in my campaign and so far it's kept player character power levels about balanced:
Restrict players to the Player Handbook, Rules Compendium and Miniatures Handbook ONLY. This applies to classes, feats, skills, spells (basically everything).
Disallow all forms of multi-classing; pick your class and when you level up in this class only.
Placed absolutely no magical shops into the campaign world.
The amount of money that shops and market stalls and merchants can offer for any magical item is a tiny fraction of what the magical item is worth.
I would recommend taking such steps in your own campaigns to keep things balanced.
Here's my advice, which might be repeated in this thread but that's OK.
Don't worry so much about whether something is unbalanced. You as the GM certainly have the right to your ban hammer, but it's the players' game too. When you heavy-handedly take away the players' toys you are telling them their fun is less important than yours.
That being said, every group should be on the same page as far as what material should be available and what system details to use. While the GM has complete freedom to decide on the material and the system, the players all have complete freedom to decide whether they want to play the GM's game. If your players are OK with three books, no multiclassing, and no magic shops, hey, go for it. But if they are used to a lot more variety in class selection, character creation, and weekly sales on bags of holding, then you are going to have to convince them why your preferences will ultimately make the game more fun (or at least stop it from being less fun). Worse, players may not always tell you they don't like how you're running things; they may just decide they have better things to do on game night and stop showing up. Some players may show up just because a bad game is better than no game at all, but that's not a situation you should rely on if you want to take the position of it's your way or the highway. You have to anticipate what they players will enjoy, and make the game fun for them as well as yourself.
When a player has hit upon a tactic or optimization that is pretty effective at trashing your monsters, villains, etc., don't look upon it as a bad thing. Players are supposed to try to win. It's what they do. Restricting or punishing a player who comes up with a good tactic or combo is like punishing your pet rat when it manages to beat the little maze you dropped it into, just a little faster than you expected.
When a player invents a good way to beat my obstacles and traps, or wants to try something really inventive and kind of rules bendy, in my experience I've found it's much more agreeable let the player do it and say "OK, you can do 'A' but first you have to do 'B' " ... or set a really high DC for task 'A' if you think it would be really difficult for an average adventurer to succeed. Or, "you can try to do 'A' but there may be bad stuff 'B' and 'C'." Don't just say, "You can't and it's impossible," if there is no law of nature or game rule which specifically forbids it. E.G. ignoring gravity Road Runner style or learning the Wish spell when you're only a 1st level wizard.
Furthermore, if a player decides he likes his pet tactic and uses it for everything, you as the GM are obliged to do your research on how an NPC or monster might counter such a tactic. If you have faith in the ability of the game designers to put together a well-designed system, then you can be confident there is some way to make the player's "golden hammer" less useful, outright useless, or even a disadvantage.
It is a misnomer to think of player characters as inherently "powerful" or "weak". Such definitions are meaningless when you realize that the most buffed-out 5th level PC one can possibly legally create can still be destroyed if you confront it with an opponent that is arbitrarily unbeatable. Because you have complete control over how powerful your monsters and NPC can be, you can always dig one out from a book (or create one yourself) that is capable of putting the buffed-out PC in his place. Now, I am not saying that is the right thing for you to do in every case, or even in most cases. This is just an illustration of the basic truth that power and invincibility is always relative. If you have a bunch of players who all want to build what they think are "super powered" characters, let them do it! Let them push the boundaries of what a beginning-level character is capable of, and let them max everything out. But when they start facing Big Bad Epic Monsters that PCs of their nominal level probably have no business messing with, they may realize the futility of trying to "game" the system.
Finally, please bear in mind I am talking mainly about tactics and combos that take advantage of rules synergies or special exceptions written into the system, and which is in keeping with the spirit of the original intent of the rules, that let PCs do a little bit more than what would normally be expected from a character of their level or class. I am NOT talking about creating invincible characters by exploiting loopholes, abusing broken or poorly-wood game rules, and completely ignoring the original intent of the game designers. That is a whole other gray area of semi-cheating and metagaming which no GM should feel obliged to agree to.
