Setting the activation to admin will drive away visitors if you're not able to staff your forum 24 hours a day. If someone wants to post, they want to post now. If they have to wait for activation, they may never return once they are activated.
***** is right - it's all part of running a forum. Spam is going to show up no matter what you do. You can delete the members when they show up, or you can make examples of them which may discourage others from making an effort when they see it will be removed quickly. Here's an example of what I like to do on my forum:
http://www.daysofblunder.com/forum/viewtopic.php?t=2087