Hopefully this isn't too late to help, but: if you're as bugged as I am by the way Yahoo!'s Instant Messenger client update notice shows up, here's how to make it go away.
Backstory: a few days ago I fired up Y!IM. A huge winky-face popped up, presenting me with two options: I could 1) update to the latest version of Y!IM, or 2) turn on automatic updates and then update to the latest version of Y!IM.
Missing options included option 3, Not Right Now I'd Really Like To Get Some Work Done, option 4, No Thanks, I'm Happy With The Current Version, or option 5, Please Quit Hassling Me About This.
Thankfully there was an X in the upper right corner--a critical UI element missing from such Y!IM features as buddy add request windows--so I closed the evil thing and went on about my business. The update notice has been popping up every now and then since then, so I thought I'd do a quick write-up detailing how to make it go away for good.
Warning: if you do this, you will disable any and all automatic updates until you re-install the thing from scratch. It's easy to re-enable, however, so don't be put off.
To do this with Windows, open up your hard drive (probably C:), and drill down through Program Files, Yahoo!, and finally Messenger. (Anybody out there with a Mac is invited to contribute parallel instructions.) Find the file called yupdater.exe and rename it to yupdater.bak, or some other non-executable extension. If you ever want or need to update Y!IM, rename the file back to its original, restart Messenger, and you should be good to go.
I do love working at Yahoo!, but certain Y! products do certain things that I think come across extremely heavy-handed. If you don't want to go to the new version of Messenger, you should not be presented with a big fat upgrade notice that looks like you have no choice. If you do find yourself on the new version--perhaps you were undercaffeinated, or a loved one clicked OK when he should have clicked the little X box--and want to go back, oldversion.com is your friend. Personally I think Y!IM reached its pinnacle at Version 6, but I'll leave it up to you.