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I'm getting some folks who've been banned showing up with IP addresses that resolve to Google in Colorado. Is Google offering a proxy server service these days? I know that there are lists of proxy sites posted to some Google Groups, but I'm getting those Colorado IP addresses which seem to indicate that the malefactors are posting via Google's servers.

Not sure if Google offers public proxy servers. However Google does offer a couple of features and services which can be used in a way to mimic a proxy server. For free of course.

And I can ban an entire range of IP addresses, right?

The built-in banned IPs filter will treat each line entered as a regular expression. So you can create items matching a bunch of stuff.
E.g.:

If your forum is set NOT to resolve IPs, then the following would ban ALL IPs starting with the number "10" as the number before the first dot:

^10\.

I may end up making a long list of IP addresses to ban. Sheesh.

Banning IPs isn't trivial, esp. if you may want Google's search engines to access your site.

s there a limit to how many IP addresses can be in the banned list?

Well .... nothing on this planet is unlimited.
However, there is no configurable limit. All depends on your server. IPs must be checked against banned list on each request to your forums and each line must be processed. So there might be some point where your server slows down.
On the other hand your forum is behind a simple, yet reasonable .htaccess protection which likely only humans can overcome. If it's just about humans then I'ld not expect any trouble with 1,000 lines of banned IP items in the list.

May I ask what your trouble/concern is. I'm sure you won't care if a human uses an anonymizer or proxy to read your forum. There must be something else to make you raise your eyebrows. What are those folks doing which you dislike. Maybe there more sophisticated actions available