I came here looking for solutions, not for opinions, but if you insist I must say I disagree with you, i allow guests to post in my forum and with good antispam measurements I have no problem with spam at all.
Sometimes people have something to say and they do not want to go thru the trouble of registering to a forum they might never come back, I know because I have been in that situation many times, i just pass on, haven't you?
I do not have problems with spam bots in my forum and guest are allowed to post. I also moderate their posts, but it is my choice and we are all very happy.
Once again thank ozzy for his help
TBH, I would never allow guests to post on my forum, but I did not go into this as I figured you were already doing this, and had no issues.