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You couldn't pause your buys or change creative? You need to have pauses in your IO and that they need to be executed immediately upon receipt. That's pretty standard.
All top tier ad servers will let you change creative without changing ad tags so, for the most part, your Network/Site is not even going to know that you changed them - especially if it's a high volume site. Also, you can use Flash to rotate different ad elements on the fly.
You also need to control the URL on your buys. Never send them direct to a single landing page. Put a redirect or rotator in between it (or even better, a multivariate platform). You can then change out LP's easily. High traffic media sources are not going to have the time or the interest in checking your landers (unless massive complaints come in, even then, if you're spending some decent coin, they may not do anything). These guys are just going to be looking at delivery, conversions and CPA to optimize.
If your first buy was with a Network and you got slap chopped then you need to do it right and target individual sites with a Media Plan. Picking sites that are targeted first and perfecting your offer there with targeted traffic can show you big time ROI.
Going to Networks first, buying untargeted remnant and expecting big returns is the most common mistake when starting off in media buying.