Currently there is no blockchain-level indicator on votes for whether they're paid (i.e. advertisements) or not. Certain bots, such as @postpromotor, do however leave comments making their usage is more transparent. I would support a "paid vote" field on vote transactions in the blockchain.

However, given that the protocol doesn't have this feature yet, it's up to frontends to decide whether to filter paid votes. The Clean Trending utility shows that it's not hard to detect paid votes at this point in time. My fear is that your downvoting operation will push vote purchasing to use more stealth methods, potentially transacting in other currencies than STEEM or SBD. This will make it much more difficult for frontends to filter paid content.

In other words, why punish producers of high quality content who decide to advertise just because the primary frontend (steemit.com) uses a trending view that doesn't filter paid votes. I feel like an ad-free trending view would solve the problem best. And in the long term, the community should consider a protocol level flag for paid votes.