I've just started using Catia (V5), and I find that when making a sketch with a face behind it, in trying to drag a selection, I end up dragging the feature and copying it somewhere >.<
If I remember correctly, you simply uncheck a box in Tools > Options to switch this off.
Tools > Options > General (in the tree) > General (Tab) and uncheck the box "Enable Drag,Drop for Cut, Copy, Paste use."
There are two icons that you can use when working with a sketch. One sets what you see, and the other sets what you can pick.
These icons are in the Visualization toolbar, normally on the bottom of the screen. Both look like light blue bricks with a white plane in the middle. One option is NO BACKGROUND that might help you. Another option is UNPICKABLE BACKGROUND.