“Are you aware you might be ruining Christmas?” is how I started, and ended, an interview with one popular ad network recently.

I’m not surprised that the companies delivering ads across a majority of websites do not want to talk about this. Delivering remarketed or retargeted ads based on the websites you've visited, and the products you've shopped for, is what they do best.

But at this time of year especially, it's likely to spoil an awful lot of surprises.

Let's say you buy a PlayStation 4 for your son on the family computer. You close all your browser windows, and junior sits down at the computer to do homework. He opens a browser session — but because an ad network's cookie has tracked your shopping, he notices ads touting deals for the PlayStation 4 surrounding every web site he visits.

Just like that, the magic of Christmas and holiday gift-giving is gone. If junior is young enough, he may wonder whether Santa is trying to send him a message or torture him: “I know this is what you want, kid, now you have to see it over and over again.”

And if he's older and more savvy — well, now he knows that the last gift mom and dad will hand him on Dec. 25 is the PS4.

Forget me never

“Ad networks are very good at remembering and very bad at forgetting,” said Dave Vronay, co-founder and CEO of Heard, a technology agency focused on content and reputation protection. Ideally, he'd like to see ad networks that remember nothing at all about your browsing habits.

Vronay may be a bit biased here, but he’s not necessarily wrong. I shopped for patio furniture a few weeks back — and even weeks after the furniture arrived, I was dogged online by ads for the exact same products I had already purchased.

“We think it’s a bad idea,” Vronay said. “Both for the end-user experience — ‘Why are you showing me an ad for something I was interested in at Christmas time? Now it’s February. How do I make you forget that?’ — and for advertisers.”

Vronay claimed it hasn't even been shown to be effective for brand messaging, but when I asked about data to back up that assertion, he admitted it was nothing more than anecdotal evidence.

There are times when ads that are driven by contextual interests are very useful. When you’re on the hunt for the best deal on Playskool's Sesame Street "Let’s Imagine" Elmo, you will want to see ads for better and better Elmo deals right up until you finally buy the furry red toy.

Similarly, restaurant suggestions, deals and coupons while you’re on OpenTable.com are probably welcome.

It's everywhere

Some of you may be thinking: “I'm okay, I do all my shopping on my iPad.” Good for you. But most online shopping still happens on the desktop. In the fourth quarter of 2013, according to online personalization services and solution company Monetate, PCs accounted for almost 75% of online shopping activity.

That number is falling — but as of the third quarter of 2014, Monetate reported PC-based online shopping was still 65.4%, a healthy majority.

And by the way, if you share that iPad you use for shopping, its browsers will also show retargeted ads that may reveal your holiday shopping habits.

It’s not just ad networks that ruin Christmas. Amazon, arguably the number one holiday shopping site, is a land mine of secret Santa shopping info. Even if your cart is empty, Amazon remembers everything you looked at and will keep pushing it all front and center — which means anyone opening Amazon on the same account will see your gift-buying ideas.

Google's AdSense, probably the leading ad-serving and retargeting network on the web, declined to comment for this article. AdRoll's representative laughed nervously at the question, and was unable to track down AdRoll execs in time for this story.

Andrew Waber, an analyst with online advertising network Chitika, told me that he worries retargeting “obfuscates user intent.” Chitika does not remarket ads; Waber said that the practice, which makes an educated guess about your interests based on browsing history, might not even guess right.

“For example, if a user's search query is ‘Where can I find parking for the Red Sox game?’ a subsequent ad is most likely to retarget to Red Sox and sports-related items," he says. That "removes the user's real intent, which is to search for parking.”

In my experience, though, this retargeting is usually spot on, which means online holiday shoppers like me need to find ways to make sure ad networks are not the Grinches who stole Christmas.

Opt out

It’s not hard. Most web browsers offer “InPrivate” or “Incognito” browsing modes. This prevents your browser from collecting your history; it also stops websites and ad partners from dropping cookies on your machine.

Without those cookies on your PC, it's nearly impossible for ad networks to intuit which ads you should see in future browsing sessions.

You can also start using different computer profiles for each family member. When you’re done holiday shopping, you log out of the computer and let family members individually log into their own accounts, which will then feature only their browser histories.

There is also the option of turning on Do Not Track in all your web browsers. This will stop virtually all tracking on websites and ad services. But to do this, you'll have to dig into your browser settings. For protecting holiday shopping secrets, though, this is probably the best we can do.

As for the rest of the year? Well, some people appreciate these targeted ads when they help them find the best deal. Heard’s Vronay, though, is not a fan. “This aggregated notion of my identity that becomes something that I can’t control and owned by these networks like Facebook and Google and I’m not sure of what they’re doing with it. That makes me uncomfortable.”

Bah humbug!