That is a risk, but it is hard to do and wouldn't work most of the time.
> Maybe that would give rise to DaaS (downvoting as a service)
There is no downvoting in this system, only upvoting. (How would you take bitcoin from someone?)
A lot of good posts and a lot of good comments are at a single-digit number of points (quite often 1), because clicking the upvote button is effort. I can only imagine how many posts will stay at one bitpoint if it takes actual money / work / computation to upvote. And those will be indistinguishable from genuine spam, trolling, off-topic rants, etc. (Given that the current Reddit uproar is about poor tooling for moderators, I don't think that shifting even more moderation burden onto the mods is fixing anything.)
There might be an exception in the voting patterns on the very large / default subs, but decentralizing the defaults alone is a very different goal. Most of the value of Reddit, especially if you care about anti-censorship, is in the long tail of subreddits.
This system would not be the same as the reddit of today. Whether moderation would still work - see all my other comments in this thread about flagging. Basically, flagging would serve as downvoting and would be partially effective. Would love to find a better way, but flagging may be good enough.
Lets just think about how many coins have been "stolen". Those are all available to crap all over your p2p blockchain forum. The normal users could never compete. So now, not only do you have to maintain enough hashing power (as suggested above), now you need to maintain a sizable wallet of btc that can outspend a thief's reserve of coins. Just so your users can post on a decentralized forum network? Sounds like a losing battle. Good luck.
In a Bitcoin economy those are also available to spend on useful things, just like money. Put it this way: would rich trolls spend significant cash to troll you? I don't think so - they'd buy valuable things with their money for themselves instead.