Online ad retargeting is a very powerful concept that, in the wrong hands, can not only infuriate your prospective customers but also threatens the entire cookie-fueled display advertising market.

We’ve all been the victim of this: you look at a pair of pink, fluffy handcuffs on BondageMaster.com and for the next three months, the very same handcuffs appear on almost every other site you visit. Spooky.

I’m the victim of exactly this kind of cyber-stalking right now, at the hands of Fuze Meeting, the nothing-if-not-tireless web meeting service. Here’s what’s happening and how you can avoid it:

We were looking for a new web conferencing service for Velocity and came across Fuze. Looked interesting so we checked it out. In fact, we signed up for a year. Big mistake.

Even though we’ve bought the damn service, we’re now all being followed all over the web with Fuze Meeting banners and boxes and MPUs. Not just on a few sites: everywhere. It’s banner bullying.

And not just for a few weeks. Seemingly forever.

Worse than this: the clients we invite to our Fuze-hosted meetings are also subjected to the Stasi treatment. So essentially we’ve become a recuriting arm for this Hydra-headed marketing beast.

That’s more than embarrassing, it’s unethical, who gave Fuze permission to look in on our meetings and drop a cookie on our guests?

I’ve actually complained to Fuze, via customer support and direct to the marketing department. No reply. Just hundreds more ad impressions. (Yes, I can opt out of the Google targeting service — but I actually don’t want to. I LIKE relevant, targeted ads. I just don’t like Hannibal bloody Lecter following me everywhere I click).

It’s more than annoying, it’s stupid. Not only is Fuze wasting money throwing impressions at someone who has already bought, it’s also serving far, far too many impressions to a single prospect.

The concept that the Fuze team (and any other marketer considering retargeting) needs to discover is: Frequency Capping, simply putting a ceiling on the number of impressions you serve to any one prospect in a given time. Makes sense. And, according to Robin Davies of Mediaplex, implementing this simple rule in your ad server can dramatically increase the ROI of your display ad campaign.

The entire display ad market depends on the consumer’s willingness to accept our cookies. If marketers abuse the privilege, the industry risks breaking this pact. Retargeting is already a bit off-putting to many online shoppers. It feels intrusive. But pushed to its limits, retargeting could easily trigger an anti-cookie law that would set online marketing back to the dial-up days. And we’d deserve it.

Lesson for the day: Don’t retarget people who have already bought. If you’re a hosted app, don’t cookie people your customers invited in to your domain. And if you’re a high-volume advertiser, put a frequency cap on it.