You'd be surprised. It is trivial for spammers to get a gmail account. Anyone who genuinely wants to contribute to a forum will have another email address, if not they will be able to explicitly email...

On phpbb boards I run the most productive things are:

1. Do not allow external links in profile of newly registered / non validated users

2. Do not allow registrations with gmail.com email addresses

3. Ensure "valid" timezone and country settings are selected by users.

L/