How should I decline a referee request for the paper which I refereed before?
I recommend against declining. You've already refereed the paper once, which means you can referee the paper again much quicker than a fresh set of eyes can.

Instead, send your original comments back. If the authors have updated the paper, send an explanation of why it's still not publishable as well. Then write in the "confidential comments to editor" box that you've reviewed the paper before, you still recommend rejection, and your original comments are attached. The editors will know what to do.