[CentOS] how to disable slow repository mirror?

Using the yum-fastestmirror plugin on CentOS 5.

The mirror that gets selected seems to be always the same one, and the  
particular one for my EPEL channel is slow as sin (often 30 kB/sec or  
less). Ctrl-C moves to the next one which is satisfactory, but it's a  
pain to have to do that every time. Is there a way to prevent a  
particular mirror from being selected?

I can remove the entry from /var/cache/yum/epel/mirrorlist.txt, but it  
will come back at some point with a refresh.

Also, since the mirror is slow, why does it keep getting chosen, for  
all my machines?

Cheers,
Tony Schreiner
Boston College