Okay, here I am.
First thing you have to remove the variants declaration from index.template.php: open it, search for:
'theme_variants' => array('light', 'besocial'),
and replace the string with:
'theme_variants' => array(),
This covers the "remove the option to select a variant.
Then, you can delete in the "css" folder the directories "_light" and "_besocial".
Actually, this morning I though it was more complex because I was thinking at another thing...