If indeed the forum have banned your proxy i'm afraid there is nothing you can do about it. It is down to the forum admins being too lazy to code the site the give your real ip instead of the proxy ip.
If it is your own ip that has been banned, then you can force a change by changing the mac address your ethernet card. (switching to usb will also force an ip change but you have said you don't want to do this so we'll ignore that...).
To change the NIC's mac, first use ipconfig /all to find the "physical address".
Then go to device manager, ethernet card properties then advanced. You will be able to specify a mac address. Type the one you found in ipconfig but increment the last digit by one- NB f does not become G, but would become 0 [zero]. The range used is 0-9 and A-F.
Reboot the modem to pick up a new ip and you should be all sorted
Finally, if it is the proxy that is blocked, you can try specifying a different one- see the articles sections for a list of ntl proxies and instructions on how to specify one
hth
MB