This is already being done, there's a cookie and heuristics in place that will give you an easier captcha or occasionally skip it entirely. But 4chan really does have a couple (and I truly mean a small amount of super super dedicated users) of bad actors who constantly spam and try to work around any roadblocks given to annoy the rest of the userbase. You cannot give them a reliable way to spam no matter what. That's why there's now many country and region blocks in addition to your standard VPN/DC IP range blocks. Plus the Cloudflare check added a couple years ago.