[Aaah, good ol' fashion Zoro bitchin'. This is a conversation Sanji can fully throw himself behind, and he does so with gusto, taking up the most put-upon tone he can muster]

Tch, if I thought a shitty pen could make him stay put, I would've tried it months ago. [Alas, Sanji's pretty damn sure Zoro would cut or gnaw his way out with enough time]

Haven't tried the leash idea yet... [He rubs thoughtfully at his chin] I'll have to give that a shot; thank you, Perona-chan.

Maybe a shock collar would do it.