I feel like the best way to implement this is to get the buy-in of people in the store. If you can get them to buy into the idea and want to get their names up on a board, if you can get them to want to buy into a sort of "store league" idea--cool, good. If it feels like an imposition, with you hovering over peoples' shoulders so that you can add their score--win or lose--to the Wall of Death, then you won't exactly be cultivating a good atmosphere in the shop. Perhaps let people have "ranked" matches and "unranked" matches, with only the former being added to the board--so if you are competitive enough to care about your score and are, say, trying out a new army, it wont reflect poorly on your stats.
The easiest way I could see this happening is some sort of competition or something. Nothing major, necessarily, but I dunno, some gubbins, a few $ store credit, some sort of gaming bragging rights, or something in that vein for the most wins marked on the board per month... obviously the little prize-thing might most often go to those selfsame WAAC players, but it'd be it'd be a way to "gamify" the scoreboard and give the WAAC players a reason to buy into the system. It might even promote more games played over the month, or something--or friendly rivalries between people high on the leaderboard.
This is just spitballing and brainstorming, of course. I do think "leaderboards as something to take pride in" is important, though, rather than "a big warning board I put up to warn people which players not to play with." It can serve that purpose, obviously, but... don't make that the explicit purpose of it.
What sphinx of plascrete and adamantium bashed open their skulls and ate up their brains and imagination? Imperator! Imperator!