Many fake user profiles may be created to generate links from the member profile page. To turn this to your advantage, edit the forum template to remove any outbound links from the member profile template. The advantage comes from activity being noticed from search engine spiders and regular new content being added to your site.

Avoid having a welcome thread. This is surely a magnet for spammers since they don't have to contribute anything meaningful to get their message published on your site. It seems very anti-social to do this, but what is the primary purpose of your forum? Is it to chat with bots?

Regularly prune inactive members, members don't need to register to read threads, and if they don't post, they may as well be de-activated. Maybe after 100 new members have registered and they didn't post anything useful for 1 month. If you leave it too long, your member list may spiral out of control. Remember that almost all of these inactive accounts will be fake users.

Delete posts that offer little value to discussions that only thank for the information rather than hitting the thanks/like button.

I think that by being quite aggressive from the start in pruning inactive accounts, it should be easy to detect when worthwhile members participate in your forum, and easy to tell which accounts are there only for SEO.