This dilemma has been bugging me for a while. I want access to our communities and tools to be as open as possible but I need to keep out the spammers and I would like to understand what the good people are doing.

As Benjamin Mako Hill says:

Should online communities require people to create accounts before participating?
This question has been a source of disagreement among people who start or manage online communities for decades. Requiring accounts makes some sense since users contributing without accounts are a common source of vandalism, harassment, and low quality content. In theory, creating an account can deter these kinds of attacks while still making it pretty quick and easy for newcomers to join. Also, an account requirement seems unlikely to affect contributors who already have accounts and are typically the source of most valuable contributions. Creating accounts might even help community members build deeper relationships and commitments to the group in ways that lead them to stick around longer and contribute more.

The issue is addressed in a paper, “The Hidden Costs of Requiring Accounts: Quasi-Experimental Evidence from Peer Production.”   by  Benjamin Mako Hill and Aaron Shaw and published in Communication Research,