The following:
/bind tab "target_enemy_next$$target_custom_next enemy alive marcone capo"
...should do the job. Change the name of the bind key and remove the initial target_enemy_next$$ (makes it do a normal target if there are no Capos about) according to taste.
Well that seems to be a good one. Thanks I will try it out so no capo will ever feel secure!
I do the exact same thing when going for the Gangbuster badge in St Martial - the keybind I use (somewhat similar to Standoff's) is:
/bind e "targetcustomnext enemy alive Marcone Consigliere"
Replace Consigliere with Capo, and E with whichever key you prefer to use for it, and it will work for you in Port Oakes.
But really if you're L50 I think you'll find it faster getting the badge in St M (or get a L50 to help you who also needs the badge, seeing as you want it for the contact). In the top right corner of the St M map ("The Flop") there are two buildings that L37-39 Consiglieres spawn reliably around, sometimes 2 per spawn. Just keep running a figure of eight around both buildings, killing Consiglieres as you go, and they should continually respawn - if at any stage they don't respawn (because you're not mopping up spawns, you're only killing bosses) then just leave the region "The Flop" briefly (moving into "Jackpot" nearby usually works, or out to sea beyond the boundary of "The Flop") and return and the spawns should have re-popped (unless there's another player lurking around that can see them - that can prevent respawn).
Usually takes me 60-90mins to get the badge there.
Thanks for the aditional info valkryst. I needed to do this on my low level char to unlock Slot Machine. Which i have done by now. The bind was surely a great help. (So was the assistance of another few players to hunt with) They know better now to leave me alone and i can go gambling. 
Binds, I just cant seem to get them working. And since the search engine seems to be on strike here comes my question about one of the most basic binds ever.
I need to hunt 200 of these pesky Marcone capo's so I can get access to a contact I have not done before. Something that must be remedied soon I am sure you will agree. Problem is those capo's like to hide and it is hard work to target them manually, worse i can miss them while traveling past. Hence the bind I need. I tried every combo I could think of but no luck so far.
So how do I create that elusive bind to make the live of the "marcone capo" impossible?