Not knowing what addons are available for this software, it's kind of difficult to answer.
On the subject of Spam Bots and Bots in general, I forgot to mention that over the years I ran forums, IF you want your forum to show up in search engine results, you have to allow them access to your forum. However, as I previously posted, you should make a Visitors forum, and install the [XenCustomize] Thread Visitors addon that IF it works like similar addons for other forum software, it could be set to scan the forum every once in awhile (but not too often or it will slow your forum down), and posts a list of currently active forum threads, but nothing in them. No posts or member information. You can even make the items on the list active links, but when a visitor (or Bot) clicks on them they get prompted to register or log in.
In most forum software, you can set each forum to automatically delete older posts, which would be handy for maintaining your list.
I wish there was a free version of XenForo so I could look at the Admin Panel and figure out a specific procedure for this...