Hello … was having a similar problem for about a month. Lots of spam posts showing up, but at least they were just in our queue as Pending, so nothing got posted. It ended up that subscribers somehow had access to all our posts (including editing, adding, deleting), rather than just being able to read posts. We didn’t have our User Capabilities plugin activated, so I activated it and then gave the subscribers the capability to only read posts and nothing else. We use Capability Manger with WP version 3.2.1 and it seems to work fine. There’s also User Role Editor.
