Random internet user would purchase an account if service provided value for them. For example, maybe they can create forums via this system for their gaming guild etc.
Yes, advertisers can purchase an account but there might be a popular moderator who bans these viral advertisements, based on popularity of ad-blocker I imagine such an moderator would get bunch of subscribers. Perhaps moderators can make some money through their subscribers as well, might be an interesting scheme.
Also spot price is pretty strong deterrent for advertisers especially if they keep getting banned through popular moderators.
I can see some users pay the fee so they can downvote opponents, but otherwise why would they? And of course they can still pay a higher fee to downvote multiple times, if they are angry enough, which is exactly what you wanted to prevent in the first place...
And unless you put a similar fee to unlock voting, you don't prevent a whole class of issues with bots.