"None of the nine tools we tested empowered study participants to effectively control tracking and behavioral advertising according to their personal preferences," the researchers wrote.

Tools for creating opt-out cookies give users a laundry list of ad networks with little or no additional information for users to decide which ones to block.

As a result, users generally opted to block all ad network trackers instead of making informed decisions on a per-company basis, the researchers found.

Another problem: the default settings for most tools were "inappropriate" because they come out-of-the-box with most protections turned off, putting the onus on users to activate and configure them.

A related issue is that the tools do a poor job of explaining to users how they work and how they should be configured, presenting information in terms that were either too simplistic or too technical.