We don't.

Now, how do we reduce, if not entirely eliminate, the amount of spam we see? Read from relays that have spam filtering of various forms.

Paid relays naturally have less spam, because spamming gets expensive fast. There are also web-of-trust relays that only show you notes from users within the relay owner's web-of-trust, which greatly reduces the chance of any spam getting through.

I would also like to see more utilization of requiring some amount of proof of work to post to a relay, but that will mean more clients need to support adding PoW to a note.

Ultimately, though, you are your own algorithm. You aren't going to see spam in your main feed if you don't follow spammers. The only spam that is a pain to avoid is reply-spam. However, having a good inbox relay that only allows those within your web-of-trust to write to it is a good start.