I don't know, i've looked around and there doesn't apear to be any easy way to do it. Sounds like an OCD thing to me. Red lines never bother me, i do technical writing and i just ignore ones that aren't valid.
There may me some sort of 'hidden settings' tool you could use to do it also, but the one i have doesn't do it.
First page of Google search i found this for disabling it,
http://dryfish.livejournal.com/8148.html
And this guy did some other stuff to.
http://fuerstnet.de/en/disable-spell-checking-mac-os-x