I'm sure they aren't prepared, but a net is the way to go, seriously--a big ol' dogcatchers' net, or a throw net. They actually just hand-grab them at the zoo, when they need to do simple medical stuff for them. Hell, in a pinch you could toss a heavy blanket over it.

Emus are big strong birds, but they aren't velociraptors. They're not as big and strong as say, horses; I don't think a cop would try to taser a horse to subdue it.