They can, but it's adding more targets and workload on to an already thankless task.

Also, downvoting isn't really the ideal solution either. It adds even more spam to the chain, driving up RC costs for legitimate use. It also gets controversial in gray areas (and sometimes not even gray areas but just disagreement with the spammer who feels they should be entitled to do whatever they want with their stake) which further adds workload (and sometimes outright harassment) to what abuse fighters have to deal with.

If downvoted consistently like that, the spammer might give up (though not necessarily right away), but also may just switch to another account or accounts and keep going. It becomes a game of whack-a-mole (which it already would be to the extent that if incentives exist, and I'm pretty sure they do, it won't be one or two spammers doing it, many will).

All else being equal, not having the incentive be there in the first place is much, much better than needing to counter, which is what the curve has done. Now all else is not equal, and there are other factors, but there is definitely a cost to consider.

Adding even a relatively small real cost to post would also kill all of this spam immediately since, as with most spam, the incentive depends on posting being nearly free which justifies the return from a relatively low to tiny success rate. But of course I doubt anyone wants to do that since it would get it the way of normal social use.