Approving members to your community can be an effective way to tackle spammers. I especially like application forms.
Approving members can make being a member feel special. They can feel rewarded for who they are. Approving members can increase the sense of unity amongst members.
Approving contributions, however, is just a nuisance. It slows conversations to a crawl. It increases the manpower burden. Members don’t receive that instant gratification from seeing their post on the community and receiving quick responses to the post. Approving posts decreases the level of social presence within a community by lack of immediacy. Approving every post will usually kill a community.
Be very picky about who you let in and then let them post what they want. Post-moderate, don’t pre-moderate. Try to remove or reprimand members, not their posts.
p.s. If you can’t approve members, then why not only pre-moderate comments from newcomers and let those who have made 5 or more approved contributions post freely?