Is this an effective area of denial weapon?
Certainly since it could allow China the potential to deny the US surface fleet the ability operate in range of the coast which is why many commentators in the US have taken the risk posed by the anti-ship DF-21 medium range missile seriously. There are a few caveates however in that China needs to demonstrate it has the ability to detect and identify US assets at range and reliably guide the warhead during the final stages of flight. The first part is not so difficult as China already has a number of over the horizon and surface wave radar assets to call upon and seems to be building up a constellation of ocean surveillance satelites ala Soviet US-A/P. What remains to be seen in fielding a anti-ship DF-21 is if China can follow the Soviet Union/Russia and the US in building ballistic missiles with accurate and reliable terminal warhead guidance.
What is left in terms of options when faced with this comparitively cheap anti-carrier weapon?
In terms of defences the US does have an embryonic defence against the DF-21 in the AEGIS system, though this is more last ditch self defence than an offensive shield. For one the SM-2ER would appear to be at the limit of its designed performance with the DF-21 and I dont believe it has ever been tested against something with that kind of performance. Also the radar of the AEGIS system would not allow tracking until the final stage of the engagement and the rapid window for engagement, limited number of missiles that can be guided at any one time or launched over a period all does not put much faith in that system being a solution.
A far less risky alternative would be an aircraft with greater endurance or a stand off missile that aircraft could carry which would allow the fleets to negate the long reach of the DF-21.