There are measures that can be put in place to reduce this but it's not as easy as most think.
These are not humans making these posts - it's automated software that runs nonstop. CAPCHAs aren't a sure bet. Remember how a CAPCHA used to be one or two words? Developers made software to get around that. That's why you now see animated CAPCHAs, choose all images with cars, etc.
You can try forms of CAPCHA, you can try IP bans, but these guys are always finding ways around it. Security is reactive, not proactive.