Rule action to remove a product from the cart? (using the commerce module)
Check what rszrama, the Drupal Commerce leader, says about this:


  If you find and delete the line item, it should work just fine. You can do this with the "Delete entity" action. You could also just unset the price of the line item you want deleted during "Calculating the sell price of a product." That's what the "Unset the price of disabled products in the cart" default rule does, so you can refer to it for an example. Unsetting the price just involves setting the amount to NULL (i.e. no value, not 0).


http://drupal.org/node/1267156#comment-4937288