@Sokrates, re: the 'only 3 lines of code'
Then in wp-rss2.php edit THE LOOP with a test for this custom field to exclude posts where the value of this new custom field = "no"
Alas, not being so handy with the PHP, reading about a test of the custom field causes my eyes to roll back into my head. With my skills (slim to none), we're talking about taking a few to several hours to come up with those three lines of code. Research into all code surrounding custom fields just to use the right name. Puzzling the example you cited to see how --how-- to extrapolate from one to the other. Such things are not obvious to me. At all. Somewhere at this point, I look at my stats again, and say, well, someone hitting this feed 200 times a day (average size in bytes, not kilobytes) isn't so bad, is it? Do I really need to do all this stuff just to get rid of that entry in my top 10 server log? Naaaah. Life is short, breathe deep, move on.
I wanted to know if there was some kinda twiddly configuration in Options (control panel) or something. Looks like there isn't. Sometimes learning more about the problem to discover that fixing it is more trouble than it's worth is a meaningful solution to the problem. Thanks all the same. :)