Ban something. Anything. Anything at all, no matter how insignificant, no matter how inconsequential, no matter how obscure. Ban it, and at least one of your players will undergo a startling transformation. Previously reasonable, rational, cooperative people will change, and it will be revealed that something horrible was wearing their skin the whole time, masquerading as an adult human:
A tantrum-throwing infant who wants only the things they are told they cannot have.
It doesn't matter how mature or reliable your group is, the second you ban something -- whether content from an entire book, or a class or race, or right down to something as tiny as a single weapon (looking at you, spiked chain) -- the hideous screeching little gremlin is going to leap out of your player's mouth, leaving their skin and clothes in a wrinkled pile on their chair, and is going to start shrieking and helicoptering his tiny dick around as he cries that he wants that one thing you banned.
He will shriek obscenities at you, tell you that you're a shitty GM, that you're stifling his creativity, or that you just hate the one thing he wants. He will cry about railroading (even though that's not really related to banning things), that it's your job as the GM to accommodate the players and if you don't be a doormat for every little whim, you're a railroading power-mad control freak.