Ahead of The Guardian Changing Media Summit today, Hornby said that media agencies need to "invest time, energy and talent" to come up with the best strategies and policies for their clients.
His comments follow research from Adloox, a third-party pre-bid ad verification system, which shows that brands could be defrauded of $16.4bn (£13.4bn) in 2017.
M/SIX, The & Partnership’s media agency, is working with Adloox to, according to Hornby, help reduce ad fraud for clients including Vitality, Just Eat, TalkTalk, Virgin Money and News UK "down to just 0.4%".
He added: "It’s good to see many other reputable agencies have taken the same stance – refusing to buy media for their clients unless they have robust pre-bid ad verification policies in place."
Hornby said that clients should also be investing in "proper brand protection" and "get themselves off the drug of cheap digital media".
He also said that Google and Facebook need to "step up to the plate and engage with initiatives like those launched by TAG and Jicwebs", the industry standards bodies.
Hornby added: "They also need to put much better preventative measures in place to ensure that offensive and politically inflammatory content – beyond the wider debate about whether it should be published at all – is not and cannot become ad-enabled."
The Adloox study claimed that the global cost of ad fraud has been "significantly" under-reported. It said that the real cost may have been as high as $16.4bn in 2016, 20% of the $66bn spent on digital ads.
The ANA’s 2014 Bot Baseline Report has previously found that ad fraud costs brands $7.2bn globally each year.
Hornby said: "These figures serve as a stark reminder that much still remains to be done in order to protect and nurture the future vitality of the digital economy.
"We have a duty to come together as an industry – from media agencies and industry bodies, to big-platform players like Google and Facebook; bringing in government help if we need it – in order to protect our own future and those of our clients."