Some quick stats to show the signup issue:

The highest user ID number right now is 6221. IDs 6217-6222 all registered after my thread creation, and are all obvious spam accounts that still got past the spam checker. Earlier this year I started deleting spam accounts after upgrading the boards to have spam checking against StopForumSpam.com. Any gaps in IDs is from users being deleted.

The most recent legitimate user is ID 6187 from 10/31. That user even posted once, attracted to one of Tony's threads. 6187-6217 shows 30 accounts since 10/31, a slow time.

Expanding to the past year, a legitimate user signed up and posted on 10/25/13 with user ID 5189. 21 legitimate accounts exist between ID 5189 and 6187. Leaving 977 spam accounts over the past year.

The effort to mass delete the accounts isn't too bad, but due to the admin UI, a single misclick is all it takes to wipe all users. I did this once last year.

So, with that said, some downsides I see to turning off new user signups to consider:

1. No new users can register to participate in the for sale area of the board.
2. No new users can register to participate in the many off topic technical support threads here. Several of these threads still hover high in Google search results.
3. No new users can sign up to lurk, and have the boards track their read and unread threads across devices.