Play Fair, Very Fair

Legendary Enchantment
Players can't spend more mana each turn than the number of lands they control.
Each spell that would cost less than its mana cost to play costs its mana cost to play.
If a permanent that wasn't played from a player's hand would come into play, remove it from the game instead.
If a spell or ability would put a card into its controller's hand, remove it from the game instead.
The first and biggest question about this is whether it would fit on a card. My guess is "unfortunately not".
The second question is whether a player's draw step counts as a spell or ability controlled by that player -- if so, the last line needs to be modified.
I think the intention of the card is pretty clear. What sort of things can you still abuse under it? Exploration, Vampiric Tutor, Volrath's Shapeshifter... what else? In any case, this shuts off the vast majority of shenanigans. Given this, it's also very powerful; just how much so, I'm not certain. Lots of colored mana in a card's cost is generally discouraged, but it's a fine tool when used sparingly. This card here is very flavorful and almost
the iconic card for White, so I think it's warranted. (It's also Legendary for the same reason). The better question in my mind is
how much white mana it should cost. I ballparked it at four for now, because Invoke Prejudice is cool.
(Also, what sort of things could you abuse
with it? The Hunted creatures and their ilk come to mind, for one...)