I block certain domains in Google's competitor ad filtering and it doesn't work. I go back to the site and the ads are still there from competitors and unrelated sites I have blocked.
Additionally Google should allow blocking by keywords in ads and landing pages rather than URLs, otherwise some companies just keep changing their domain names and putting up new URLs.
Google should provide a way to do run ads in such a way that the competition cannot basically steal traffic from a site by sending a bunch of fake hits and increasing the cost of ads and using up the allotted hits per day with traffic that will never lead to a positive conversion rate.