I use WPMU's "Comment Indexer" to compile all the recent comments on my multi-site installation. I set up a private page and use the "Custom Sidebars Free" plugin to place the "Recent Global Comments Widget" on that private page.

Every morning I take a quick look at my private page, scan down the list of comments, right-clicking the spammy-looking ones to open in a new tab. Then, I edit them to "spam" status.

As part of a first-thing-in-the-work-day routine, it does a good job of keeping the cruft from appearing on any public pages.