How are you determining that the person is getting past?
Is it because they are doing something that gives it away as being them (forum trolling, comment spamming, etc...)?
Or is it because you are seeing an IP accessing your site that shouldn't be?
When you said:
I took it to mean that someone was still accessing it from the IP... not that the same person gained access, but from a different IP.
Sure, you can get around IP blocks by using proxies, but that's just because you're accessing it from a different IP... so the IP you're blocking wouldn't show up anywhere.
That's the downside of just blocking IPs, especially considering the endless list of proxy sites.
If you're just dealing with one troll, you could be adding single IPs until he gets bored and gives up.
You might look at other ways to block, if it's a problem with the content that's being posted. Depending on what the site's running, and what the person is doing, maybe blocking certain content could help?
I've never looked into it, but you could probably find a list of known proxy IP ranges and just block them. There's always a way around, but someone might get bored sooner if the common proxies he's trying are already blocked.
Another option if it's just a forum, blog comments, etc... would be to manually approve new registrations for awhile until he goes away.
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