Helping people with computers... one answer at a time.
One the most common classes of questions I get has to do with people who've lost access to their own accounts because someone else changed their password. Setting "remember me" on a public computer is a quick way to set yourself up for having your password changed and your account stolen.
I laud your efforts to help prevent this.
Sadly, the news isn't all that good.
I could find no official way in either MSN Instant Messenger, or Yahoo Instant Messenger to disable this feature. One would think that with the rampant problems it can cause, there would be a documented way for people in exactly your position to so.
However, I didn't stop there.
Unfortunately, I could not come up with a way to coerce MSN Instant Messenger into disabling the feature. Similarly, with Yahoo Instant Messenger, I thought I had a line on a registry setting that could be tweaked, or perhaps permissions on a registry setting could be altered to prevent it from being changed, but alas ... my experiments failed.
I'm hopeful that perhaps a reader will report back with a hack I've missed, or perhaps even an officially supported mechanism that I overlooked.
But for now, you'll have to rely on your clients to "do the right thing" ... which I know you can't really count on.