Unfortunately in a worst case scenario this gets very political, if we do something about it in advance an impactor probe should do the job, if we decide to play the 1/50 odds and lose then the most effective short notice method is a nuke. Not a direct strike but a near detonation which vaporizes a section of the surface of the object with the outgoing plasma effectively functioning as a massive thruster. Actually doing this is not trivial but not hard either (from an engineering standpoint), the tricky part would be actually managing to launch it without every nation on earth that happens to have a beligerant leader saber rattling and stone walling the prospect of a launch until its too late even for a nuke to do any good.