I think you cant block meebo by any mean, but you have to watch out for meebo - it uses the https ports also. So, if you’re running a transparant proxy, you have to specifically drop the 443’s to https://www7.meebo.com through www36.meebo.com (and they’re may be more)…
As you can see, others have other ways to use IM messages without depending on meboo as well which is called "Gadget". you can take a look at my blog http://ezones.blogspot.com, where i embeded yahoo chat without downloading IM tools or using Meboo as an example.
So, to some extends, IM has been an important part of business communication, and still growing strong. If companies smarten up, they would see the added value of IM in the workplace and maybe they even save some $$ on phone bills.