Behidden.com, like any other web address, has assigned to it a range of IP numbers, of which you are assigned one. And that IP is what network 54 sees when you browse JOn's board. Jon told N54 to block you ISP (webtv?) from viewing or posting on his board.
So you went to a web based proxy service, behidden.com. OK, now network sees you as having a behidden IP number and not a webtv IP number. So it lets you in. But now all jon has to do is go to whois and find out the range of IP numbers assigned to behidden, and tell n54 to block that range, too.
What you need is a regular computer. Because then you could use one of the many non-web-based proxy servers all over the place. Most of these are just poorly maintained web servers who are outed by scripts, and then their IP numbers are listed on various websites, and if you have a configurable browser, you can set their IP as your proxy. I don't know if webtv will let you do that; I doubt it. But you could look around in your menus for a proxy server configuration setting.