In all likelihood, simply changing the IP address will be sufficient. Banning someone from a website is a little silly and largely pointless because it is trivial for someone to bypass that ban. It's usually better to limit functionality to newcomers, as Stack Exchange does.
Factory resetting your device is largely pointless. Sites cannot directly determine a connected device's MAC address (but can potentially do so through a court order or via hack, bribery, or blackmail).
Serial numbers, except in special circumstances, do not come into play at all.
What can come into play is device fingerprinting. Every device has a fingerprint, some more unique than others. Generally speaking, the more unique settings/software/fonts/hardware your device has, the more unique your device's fingerprint will be. Factory resetting will have no effect on your device's fingerprint if you install the same software and configure the device with the same settings.
A larger issue, however, is to take a close look at why you were banned. You reported you have now been banned from "a couple of websites like Reddit". The fact that you have been banned from multiple websites indicates that your behaviour is likely unacceptable to the communities from which you have been banned. Spend some quiet personal time thinking about why you want to be a part of communities in which you are not a valued member and in which you are likely disturbing others. Occasionally, bans happen because petty small-minded power-hungry people intentionally seek out and obtain the power to impose bans on others. But if you've been banned from multiple sites, respectfully, there's a real good chance that it's time to take a long and thoughtful close look at the person you see in the mirror.
