Lets say
[01:12:44] Immortal [thend@AC09CE.3369E6.07F5D3.BF71D1] has joined #Chat
The domain is the first 2 parts of the full host mask in this case it is:
AC09CE.3369E6
So what we do to ban it is:
/mode #channel +b *!*@AC09CE.3369E6*
Remember banning a domain results ban on a lot of people :P