Thank you dstiles, I'm just blocking via htaccess on the UA pipes at the moment. I scan my access logs daily, and I'm tired of seeing all those pipes 403s! It's just so rude, and inefficient of Yahoo. Good riddance to them.
Thank you for your kind suggestions, I'll see which will apply best in my case, and your notes will help others.
Funnily enough, I never even considered using robots.txt and it certainly seems to be the Y! pipes bot itself, so I'll have a try, and see if it obeys robots.txt (some hope!)
Nonetheless, I'll give it the chance: Just one!
We've long since blocked Feedburner, because I never liked their combination of feeds and email delivery.