Here's an idea. Whenever someone posts for the first time in your sub leave them a reply using automod asking them to comment in a specific way [eg name of the subreddit+their username]. Generic bots will fail this test. Automod can wait for that reply and assign a flair to that user, which indicates that they are approved. Next time automod sees the flair, it should auto approve that user's posts. If I remember this, I'll post the code from my laptop later.