Raven Crowking put it best in the other thread: When you have a thousand options, and you pick and choose for each campaign, the variety is endless. But when you incorporate everything, you have the same thing over and over. (Not exactly how he expressed it, but you get the gist.)
It's the same in the theatre, where some actors feel that direction "limits their creativity" not realising that being directed can actually be very liberating, and it doesn't need to be seen as restrictive.
It can be very daunting when asked to come up with something without any sort of guidance. Despite being able to go any which way you like, you can feel lost. Once given direction, that changes - you may not be able to go in any direction you like any more, but you still have an infinite amount of ideas along a particular axis to explore, with infinite variations to study. Any improviser knows that it can be hard to just come up with "an idea" on the spot, but a little direction, such as "tell me why your character wants to know what is at the bottom of the well" makes things a lot easier!
I think part of the answer is to frame negatives in terms of positives. As an actor I had difficulty when told not to fidget, but was fine when I was told to find stillness. To put it another way:
don't about the pink elephant in your pants. In short, it is not easy to
not do something.
So, instead of telling your players that they can't play a certain class, tell them that they can only play certain other classes. Or play classes with certain qualities. Always focus on the universe that is there for them to explore, or they will always be tantalised by the forbidden fruit. Instead of telling them they can't play psionicists, tell them that you are running a game where everyone is playing a martial class because you want to run a version of the Seven Samurai. Challenge them to find a way to make a fighter interesting without recourse to multiclassing etc. You might be blessed to GM a party with a career soldier, a peasant hero, a surly bounty hunter, a swashbuckling dandy, and so on.