Amazon (AMZN) is putting an end to what was probably a lucrative side-business for some of its ad partners: The company will no longer let participants in its affiliate marketing program to take out pay-per-click ads on sites like Google (GOOG).

Here's how it worked: Amazon's partners would take out a keyword ad on Google that leads to Amazon for a dollar per click or so, and if someone uses that ad as to actually complete an Amazon purchase, there could be an upside of a few bucks on the spread.

No longer: Amazon is giving its partners until May 1 to take down the ads or have their participation in the associates program terminated.

The quantity of ads is probably negligible to Google. But the plan makes sense for Amazon: The e-tailer can determine which keywords ads are working, and can improve its margins by taking out the middleman.