Make a junk chat channel that is small and doesn't get in your way. Filters -> Mercenary Messages. Put a lot of other stuff in that channel too.
I filter merc spam to a hidden chat tab instead of outright turning their messages off, because mercs also announce when they fail confidence checks and I feel that's important to know. The fail message is "More have attacked us! This doesn't look good!" if you want to make an audio trigger like I did. Notice there's a double-space between the sentences, in case the forum messes with that formatting.
There are actually a couple more messages, not just that one. I can't recall what they are offhand, as I never really see my mercs do this, but I do remember during TBL after a patch my merc suddenly started doing it a lot, I logged out and back in and it stopped. No idea what was up with that.