Quote from some comments in the question "Spam post should not be upvoted!":
Don't downvote spam. Just use the spam flag, and not another one. It may seem counterintuitive, but downvoting can make spam stay around longer in the system. The spam flag is also special.
If you downvote the post and flag it as spam, it may actually take longer for it to go away. When the vote count hits a certain point, it will disappear from the home page, and catch fewer flags, which means it will take longer to get autodeleted and may stay around until a moderator can handle the flag.
I wonder what the real problem is with downvoting spam, also taking into consideration that this is what shows if you mouse-over" the downvote button:
This question does not show any research effort; it is unclear and not useful".
That mouse-over message is exactly why I think you should use that button. If doing so confuses "the system" (does it?), then just fix the system ... Or adapt that mouse-over message to not "confuse the downvoter" ... Or adapt the algorithm that makes it disappear from the homepage so that it does not disappear if it also has some number of spam flags (so that it can continue to catch more flags).
And having spam disappear from the homepage, isn't that a good thing? Even if it is at the price that a moderator needs to do whatever extra action to really delete it.