Captcha solver are not the real problems. The problems are the proxy addresses that hampers the advertisement revenue.
Is it really hard to block known proxies, VPN's and TOR exit nodes?
Metrics analysis and WHOIS of traffic could give ppl some indication what percentage of their traffic is legit.
What might be tough to identify is bot zombie traffic. A better captcha could be the best method of blocking those since they would look like normal traffic for the most part.
It could help to know OCR's probably aren't in play.