The universal symbol for 'No' is just a circle with a diagonal line running through it from the upper left to the lower right. You put this over anything that is not permitted... like 'No Smoking' or 'No Entry'. This shape should be applied to any color of full screen layer. It will automatically fill with red, bevel the alpha and add a drop shadow. Scale as needed.
Download the 'No' Symbol' FFX file (zipped).
To install: Drop it into your Presets folder in your Adobe After Effects application folder. Refresh your Effects and Presets list if needed, by choosing 'Refresh List' in the flyout menu on the Effects and Presets palette.