Try this (untested):
on ^*:text:*:#: {
if ($nick == SOMENICK) { echo -tlbmf $chan 00,12 $+ $strip($1-) | haltdef }
}
That will remove any color/bold/etc that the person uses, but guarantees that the line color will always be there. If you want to let the person use color/bold/etc at the possible cost of losing the line color partway through the line, then you can use this instead:
on ^*:text:*:#: {
if ($nick == SOMENICK) { echo -tlbmf $chan 00,12 $+ $1- | haltdef }
}
Replace SOMENICK with the person's nick, and adjust the color codes to whatever you want. You can also adjust the echo switches if you don't need/want everything that is there.