I feel like reddit could spend the $10,000 to just buy all their services and then ban the accounts/IPs that they see.

Good vote spamming business will be completely automated. They'll have accounts in reserve pool, with ability to spin up thousands more in a couple minutes. Most likely they make a couple hundred accounts a day and prime them with VA's with real content & votes with the intention of burning them with fake votes once aged appropriately.

If you run a website and you're getting spammed, the worst thing you can do it also ban the accounts as this give spammers feedback that the account is no longer valid to use and they'll rotate it out. Best thing you can do is /dev/null the account. To the user it will look like account is working as expected, but on the back end anything they do doesn't actually do anything. Harder for spammer to know when that account is burned. This way they'll continue to use it, and you can use this data to find related accounts and /dev/null them as well.