How to permanently disable resume on El Capitan

Notice: This is not a duplicate! Other questions had to do with older versions of OS X or had to do with temporarily disabling resume. El Capitan has made my iMac very, very slow. A hard reboot should clean the slate so I can recover from OS hangs, but now windows and programs that were open when I shut the iMac down (with the power button) are restored to their previous states. How can I disable this completely? I do NOT want to have to select a button every time I shutdown/restart, which is something I cannot do if I use the hardware reset.