Your competitor has the capability of blocking you from buying ads on their site (it's part of the adsense interface). If they're not, I would say fair game. Ho

Your competitor has the capability of blocking you from buying ads on their site (it's part of the adsense interface). If they're not, I would say fair game. Honestly, if they're making money off of your ad they might not care! It's a bit dirty, but I wouldn't call it unethical.