They still have quite a few options, but to them, a hefty fee may be the surest method of weeding out bannable maps.
- They could select MvPs. - They could have a tiered permissions system. - They could have something like the Steam Workshop, where maps are voted/sponsored into rotation. - They could charge for individual uploads. - They could charge for access to the editor. - They could only pay attention if bannable maps gain popularity/notoriety (like they have with SC2, and that's worked fine so far). - They could verify uploaders with snail mail (this verifies your address, but they would still need to ban you). - They could screen uploads monthly/bi-monthly (kind of like a monthly contest). Would be nice of selected maps get some kind of reward, but this costs them money on a periodic basis. - And many moreee...
Ultimately, what they will want is something to discourage this behavior, because there's nothing physically preventing you from uploading a swastika.
To expand on the idea of a fee to access the editor, they could have developers put a hold on your credit card (not a charge), that only charges you if you get banned as a punishment fee. This way content creators don't need to pay for anything, unless they decide to break rules. There's no real money to really be gained from charging for access to content creation, so this makes sense solely as a mechanism to punish offenders. They'll still need to react when people with deep pockets decide that breaking those rules are worth the cost of punishment. There may be an instinctive reaction to putting people's credit on hold, so other forms of temporary payment are possible.
On the other hand, if they want actual prevention, they'll need to screen submissions somehow, which also makes sense if they're going to do competitive map rotations.
+ The objective isn't really to stop 'map spammers'. It's to prevent the addition of questionable content. A person uploading questionable content isn't adverse to being banned, so it can still be uploaded. However, you're not wrong in suggesting an initial player investment. Maybe they could require that the account have at least a few $ spent on Blizzard products before they can upload (skins, characters, other games, etc). This way the access fee doesn't really feel like a fee, and we don't have to 'grind' our way to accessing the editor.
If SC2 required X hours played before you're allowed to publish maps, it would definitely have weeded quite a few publishers out.