Your ISP does this. If they keep handing you the same address, it's probably because the MAC address on the WAN is the same, so the system see's that you already have a lease, and gives you the same one.
You COULD try changing the WAN MAC and see what happens. I don't know that it will fix the problem though, depending on what kind of service you have and who your ISP is. Many ISPs talk to the modems quite a bit and know that your router is behind the modem they've assigned you. For instance, I have Time Warner Cable, with Roadrunner Internet service. I have an SB5101 cable modem. When I try and get an IP from RR, they can tell that the request is coming from my house, and from a router that's been assigned in a system as mine. If they (my ISP) wanted to, they could give my router the same IP every time, regardless of the WAN MAC address of my router.
The short of it is that it's probably beyond your control, and whatever you did to get banned from 4chan is your own fault, and now you have to deal with it or learn to use a proxy. Good luck.