1. Make it possible to disable the the URL field in comments. This is where most of the spammy urls are being entered. If I'm going to allow comments from non-registered users, I may not want to ask them for their url.

Already added to Basic antispam plugin. It doesn't hide the input field since b2evo doesn't have a hook for it.

2. A setting to make comments only open to registered users.

This feature is there since at least v2. Check the Basic antispam plugin

Get the latest revision and reload plugin events after update
http://evocms.cvs.sourceforge.net/viewvc/evocms/b2evolution/blogs/plugins/basic_antispam_plugin/_basic_antispam.plugin.php?revision=1.39&content-type=text%2Fplain