There is no such unicorn as voting against. If you can produce a presidential ballot or any other in an election between people with an “Against” box on it, please do.
Easy enough.
Trump
Cruz
Pick one. If I pick Trump I am voting against Cruz, conversely if I pick Cruz I am voting against Trump. The only way I can not vote against one or the other is to not vote. If you want a none of the above option, don't vote let someone else do it for you but that does not relieve you of the responsibility to do the right thing. If Ryan survives the Primary and you don't vote for the Democrat in order to defeat him, then you essentially have voted for him despite your protestations that you didn't.