(4/11/15) Apple has published a support note describing how to remove adware.
A family member attempted to download and install a game on a Mac laptop. They picked a game that was loaded with the Geneio adware.
They noticed several problems:
The software was downloaded from Softonic, a popular download site. Much comment on the Web suggests that either they are deliberately propagating adware, or they have been hacked. Some Mac users report that installing any download from Softonic also installs a "Softonic Toolbar" and changes how Safari and Chrome work. You can apparently get stuck with this adware if you download sports streaming software also.
I found instructions on The Safe Mac that helped me completely remove the unwanted software. It had really burrowed deep into the Mac: a special startup item was installed that ran a hidden background process that invoked some well hidden libraries.
Another adware program called "Cinema Plus" is out there. It also gets on your computer if you install "free" software. It burrows into your browser and underlines words on web pages. Hovering over these words pops up ads. Here are instructions for removing it using Adware Medic. Check your browser extensions for "Omnibar," "SearchMe," "VidX," "MacShop," "MacSave," or "MacVDx" and remove them, and reset your default search engine and home page in the Preferences.