Japanese phrase
反則だよ。 han soku da yo
Meaning
That's against the rules.
When to use it
Marks a move as breaking the rules. It is common in games and can sound mildly teasing rather than harsh.
Example dialogue
サッカーで手を使っていいの？ sa kkā de te o tsuka tte i i no
Can we use our hands in soccer?
サッカーでは反則だよ。 sa kkā de wa han soku da yo
That's against the rules in soccer.