This is an already solved problem. Comments can be flagged, taking rep and rewards from the account, and if the rep gets low enough posts and comments will always be hidden by most interfaces, including steemit.com
This is as true for bots as it is for human spammers, the blockchain makes no destinction. Posts are rate limited to one every 20 seconds and no limit per day per se, but I did find this
However, there is a soft limit to the amount of blog posts you can publish before the payouts for the account become diluted. Each account can post four blog posts per 24 hour period and get 100% payout for each one. If you post five blog posts in a day, the payout for the 5th falls to 65% of the amount it would have otherwise paid.
6th = 45%
7th = 33%
8th = 25%
After editing a post, the 24-hour clock for that article resets.
The above redacted as no longer relevant since HF 19
The limit was actually raised to encourage more posting!
Since I was the one who wrote this post, I'll reply here for clarity. While this decreasing rewards curve used to exist, it is no longer in place.
Thanks for this, I didn't realize it was dependent on the square rewards curve
If I remember right, this 4-post "limit" was removed before HF19.
Yes it was