He probably snapped that shit off with the help of Hardening
Didn't really think it would cause him trouble for too long.
Wait, is it that simple? I don't buy it. Those strings are strong enough to cut stone effortlessly, to slice meteors, to keep the likes of Jozu frozen in place and to create indestructable, inescapeable Birdcage.
And Luffy just snapped them off screen?
That would be hard to do with ordinary string, and Doffy's strings are all but ordinary.