I am just doing some homework and I can't make the sentence I want.
Concretely, I want to write
"If you start smoking again, I'm going to give you a good spanking."
If I say this in England or other parts in Europe (in a different language) the person I am telling this will laugh (or at least smile). Because of course I am not actually going to beat them up even if they start smoking again.
How can I say this in Japanese?
There are so many ways to render it with a similar effect. Idiolect could play a significant role, reflecting one's generation, gender, personality, proclivity to use humor, etc.
The addressee's reaction could depend on HOW you say it, too, e.g., with affected seriousness, with subtle cuteness, a deadpan tone/face, furrowed brows with eyes smiling, etc.
Just a few examples that come to mind: また吸い始めたら +お仕置きするからね +ただじゃおかないからね +さらし首かな +縛り首（しばりくび）だな +ギロチンだね +死刑だよ +おへそガリガリの刑だからね（dozens of variations are possible on this 〜の刑).
For added emphasis you could repeat the noun part immediately after these, e.g., ギロチンだね、ギロチン！(with or without the exclamation mark).
