Welcome to Smogon! Take a moment to read the Introduction to Smogon for a run-down on everything Smogon, and make sure you take some time to read the global rules.
A Cake Wearing A Hat I hadn't actually considered neutral natures, but the rule explicitly says all natures must boost a stat, so neutral natures would be banned.
All Pokemon must have a nature which boosts a stat with no EVs
Legal Example:
EVs: 252 HP / 252 Atk / 4 Def
Jolly Nature
Illegal Example:
EVs: 252 HP / 252 Atk / 4 Def
Adamant Nature