I don't think a referral program would bring much spam.
Why do you not want bots? I think bots will be an essential part of steemit's success.
New referred user's (whether human or bot) won't hold much weight until they add funds, make quality posts, or are able to upvote quality posts beforehand.
Auto-likes won't be rewarded well because they are not going to be able to only like the quality posts. They way the system works is if a user can upvote an article that later receives high quality/volume, then they will benefit. If they can't pinpoint these articles, then they shouldn't be able to game the system.
I'm sure auto comments would get downvoted unless if they add quality to the experience.
I think it would only avoid fake account, but not fake Upvote and comments.