How do we prevent the occasional bad upvote?
We don't need to.
That question was fairly quickly closed. At time of closure, I saw that it had only one positive upvote, with however many mod flags and vtcs.
Perhaps - as I speculated in a comment - someone had seen the previous iteration and voted it up for the humour of it being reposted under the correct tag that time around? Or perhaps it was out of sheer bloody-mindedness. Or perhaps there was another shill account (that was going to do a spam response to that bait) with sufficient rep to upvote?
Without meaning to sound condescending, the usual approach of "edit, comment, downvote, flag, vtc, bring it up in Ask a Super User moderator" (delete as appropriate) works just fine :-)
Upvoting isn't worthless, as there is only a limited amount of votes per diem in any case. If someone wants to throw away valuable votes, let them- they'll quickly be drowned out by sensible folks voting the 'right' way.