Yup they go by IP however you'll need to register a new account as the post restriction is on the account. You can either find a free proxy server or you can also use a VPN, but your best bet is installing Tor and go through that as it has many exit points so almost always you connect via Tor it changes you ipaddr so its hard to distinguish based on that.

There are other techniques that you can use or they could use to distinguish that it is the same person, like using the same device which may change IPs bit your hwaddr (MAC) remains the same, you can spoof that too relatively easy. If you have a DHCP provider you can expire leases and request for new IP as well. If you'd like to go on a discovery expedition on which protocols of restriction they are using, let me know and we can probe it.

Just FYI I think a while ago law came out that if you're trying to circumvent a restriction on you on some site it is illegal, like ban on mac and you spoof it, ban on ip and you change it etc.