Bitcoin originally evolved from the "hashcash" proof-of-work system, which was intended to be a scalable anti-spam measure (force an attacker to generate a unique proof that is expensive to generate but cheap to verify). And you are basically describing proof-of-stake (force participants to stake, and if they misbehave you slash their stake). I wasn't kidding about "nothing is new under the sun" ;)
In general though others are right that the problem often is that attackers are more willing to play these games than individuals - you are not willing to put up 10 bucks to post on a web forum, but an attacker might think $10 to get a wonderful commercial offer delivered to 1000 people sounds really good! You won't spend 10c to upvote each individual song, but the RIAA will! Etc. It needs to be highly asymmetrical, and ideally have minimal/zero cost to "honest" users with an exponential penalty for attackers.
Obviously we are not living in a post-email-spam world unfortunately, and what we have is basically the "lightning network" with gatekeepers offloading and centralizing a lot of the problem so users don't have to deal with it. But you aren't the first person to make the observation that these are related problems!