Steem has several challenges that traditional forum don't have.

1. Anyone can broadcast a valid transaction to the blockchain.
2. The goal is to be censorship resistant.
3. Anyone can create a blockchain account.
4. You cannot ban by IP

That said, Steem has some advantages that other forums lack.

1. Existing spam bots are not designed to creates and sign transactions.
2. Spammers must invest in the network with VESTS to transact
3. Every user is a moderator with financial incentive to pick up the trash to maximize the value of their STEEM.

Large investors in STEEM have financial incentives to run bots that automatically vote down posts from known spammers. This creates a kind of shaddow-ban.

Lastly, the steemit.com rendering of blockchain state can automatically minimize / exclude posts with net-negative votes. If the post is shown, links can be disabled.

The user interface will also allow users to follow others. In this way posts downvoted by friends are automatically hidden in the user interface.

Ultimately all the same techniques used to prevent spam on traditional forums can be applied here. The only limitation is that the blockchain never forgets and never deletes posts.