I tried the "post before registering" once, but switched it off after a couple of days due to the spam entries in the records. I laud its intended idea.
Personally I've taken a very different approach in the past year and it's high barrier, not low barrier
Visit my site and you get a fairly blank/minimalist home page.
New member applications are only taken one day a week.
All new registrations are admin approved and have to fill out a special profile field correctly first to help pass validation.
Erased from Google. Erased from The Wayback Machine.
"Word of mouth" beats any potential "drive-by" traffic.
So a high barrier, but I believe that the future of specialist communities (that forum software encourages) is security and privacy rather than "lowest barrier/truck them all in". Though I suspect how effective that is depends on how established your site is.
The result is no spam, anyway (and a relative haven for users, who increasingly value privacy).