The GOP should wait until closer to the election and make the decision if they think Hillary will win. If so, they should accept Obama's nomination, but if they do not, they will block Hillary's nomination and let the Supreme Court sit with an even number. Another left-leaning Supreme Court Justice would switch the balance from previously conservative to more liberal.
No, the GOP should not block Clinton's Supreme Court nomination based on partisanship (which is implied by the question, since she has not yet made a nomination to be judged on its face). Blocking a justice nomination for any reason other than inability to serve in that position is a violation of the duties of the Congress, as set out in the US Constitution. In such a case, it could be argued that the Congress member should be barred from serving due to failure to do their duty.
Clinton has the rights to choose any of her choice if she is elected as President on November 8, 2016 Election. She knows what best for the people of America, if the republican plans to annul any change by her, then true judicial is lacking. As the head of states, she has the power to vote in her choice.
No, the GOP should not block Clinton's Supreme Court nomination because it is their responsibility as a political party to allow the president to choose which Supreme Court justice goes to the court. It would be unfair to block a nomination simply because they do not like all of Clinton's positions.