Telling OM to "leave your W alone" is a waste of energy and breath. If he's having an EA - or a PA - with a married woman, chances are he doesn't have much of a moral compass ... or, at least, if he DOES have one, he isn't being guided by it right now.
If you want to send a message (many on this site would encourage you NOT to, btw, and that would be more in-line with MWD's philosophy than what I'm about to pitch), there's a better idea. And this is what I did w/ my H's (now-X)OW ... after I had already texted her a plethora of snapshots of her sexual text-conversations with H ... oh, and his sexual text-conversations with ME at the same time, you know, just for good measure. (But keep in mind this IS NOT encouraged by DB principles and I only chose to do this *after* I had LOADS of evidence of an actual PHYSICAL A.)
Lock eyes with OM when he's in the building next time. Not AT ALL in a threatening way. Keep your mouth shut. But just lock friendly/competitive-ish eyes with him. Smile at him. And KEEP smiling at him. Like, not a warm, friendly smile. A sh!t-eating grin. Make it reallyyyyy awkward ... for HIM. Go so far as to wink at him if you're in the mood.
If you want him to squirm a little - while YOU keep the upper-hand - give him a subtle message like that: One that says, "I'm fully aware what you're doing with my W, brah. And I ain't worried about it. Game on."
Keep your dignity here. Stay in touch with your confidence. And trust my words: Whatever you do will get back to W as long as W is in touch with him. So "kill two birds with one stone." If she's anything like me and most women I know, she's going to be A LOT more "thrown" by ... and attracted to ... a man who exudes QUIET confidence than one who tries to be controlling and "threatening."
And, obviously, don't tell your W you saw OM. Or anything of that sort.
Confidence. Confidence. Confidence.
I can't stress that word enough.
M: 40 H: 44 Married 14 years S11 & D6; D20 & D19 from previous M 2BDs/PAs, 8 years apart Piecing: April 2014