I put a bright green padlock through my rear derailer. Its large enough that its impossible to rotate the crank around it. Its intended use is as a detterent. Its a nuisance for thieves. But its not the only lock I use. A D-lock and thick cable lock are also used. I never leave my bike not locked to some thing unless its totally unavoidable. Even then, I make sure the bike is within sight and 10 metres of me.