A filter for some domain link ?

Hi All.

Ok... Imagine I would like to ban some links to an external site on my wpmu...

Let's say a user write an <a href="domainwathever.com" link, and I would replace any attempt to place such a link for another link of my taste...

So, any link to "watheverdomain.com" would be replaced by "linkofmychoice.com"

Anybody know how to make a simple filter for this ?