The questions:

"r u there?"...
"u c amy winehouse on youtube? WTF!!!!!!"
Etc.

The answers:

1) The rebuff charitable: "Sorry, I can't talk now -- try me after hours."

Deft and to-the-point; a response for all seasons. You communicate that you're not a jerk, but that you're busy and don't have time to waste on someone else's foolishness.

2) The rebuff signification: The Away message. Begin setting your IM status to something appropriate when you're swamped. Here's a good place to start: "I'm busy, and I'll get back to you when I can."

Even better: hotkey this, if your IM client allows -- along with "in a meeting" and "on the phone" Away messages. And set an "Idle" message to indicate when you've stepped away. IM presence -- and IM itself, by extension -- is only useful if it's accurate.

Consequently, someone who needs something urgent can ping you and get an answer immediately. Someone who IMs without paying attention to your status in their IM Buddy List gets an immediate response that your busy and may not answer. And anyone who persists in IMing while you're not responding and still listed as Busy, well, then, they've given you grounds to take more aggressive action. That is, passive aggressive.

3) Ignore... with Extreme Prejudice (the rebuff exclusionary)

AKA the White List. Although you run the risk of missing the occasionally critical message from an unlisted contact, a still more aggressive posture may pay off when you find yourself inundated with useless messages. By this, I mean setting your IM client to accept IMs only from contacts already on your Buddy List -- and trimming your list to include only those who don't waste your time.

For some -- myself included -- the approach may be too risky: There's always the chance that an unlisted contact with urgent information for you might not be able to reach you.

I get IMs from new, unlisted contacts all the time -- admittedly, they're chiefly PR people, many of whom are trying to pitch me the story of the latest top-tier stealth-mode Web 2.0 solutions provider, and who have been stymied by the effort of trying to reach me via phone or e-mail.

Still, there have been enough cases where an unlisted contact had key information for me, so I'll remain reluctant to "white list" my Buddy List.

Of course, this means that you continue to allow yourself to be exposed to idiotic IMers.

4) The rebuff direct: "Sorry -- I don't have time to chat."

5) The snub supercilious, a measure of last resort for those who, so to speak, don't get the message:

"Would love to chit-chat, but you see, I actually have a job to do."

Alternatively:
"Does your boss know how much time you spend not working?"
"Are you employed?"
"Why are you even allowed to use a computer?"

...and so on.

Warning: Frequent users of this approach may experience side-effects including disapproving stares in the employee cafeteria, a key-scratched paint job in the parking lot, and internecine, interdepartmental hostilities. A small price to pay for enforcing intelligence and efficiency in workplace communications, I think we can all agree.

6) A broad technological solution: namely, granular presence controls. Certain enterprise instant messenger products offer employees the ability to specify their availability to receive IMs (as well as phone calls or videoconferencing invitations) to certain others or whole categories of contacts.

Unfortunately, this is beyond the capability of most consumer-grade IMs, which is still what most of us are using (alas).

Maybe one day, when the dreams of Microsoft, IBM Lotus, and everyone else playing in the unified communications space are realized, we'll have enterprise-grade IM with granular presence controls. Until then, all we can do is satisfy ourselves with the technological solutions currently available to us (the Away message and White Listing IM contacts) and try to enforce the lessons of workplace IM etiquette as best we can.

UPDATE: Yes, I've written about smart IM practices in the workplace before, in 2004. People still haven't wised up, sadly.