IPv4 range is single to three digits, in four blocks.
0.0.0.0 to 255.255.255.255 *to keep it simple
Assuming someone has a dedicated IP and they're the only ones using it: 213.23.45.187 would then be that individual's modem at home using the assigned IP. (or whatever the setup situation is)
If you ban 213.23.45.*
Then you ban 213.23.45.0 to 213.23.45.255, blocking out potentially up to 254 others.
Of course, if someone has a dynamic range, such as 213.23.45.*, and despite an individual ban they return and return, you can limit his options by using * (wildcard) to cover that dynamic range.
Note that if you ban 213.23.* that you ban every combination of 213.23.0.0 to 213.23.255.255 => that's a damn lot of potential users that most likely have nothing to do with this one individual.
Note that a user that's abusive and doesn't take the request to no longer visit the site (if they dont like it, why bother right) is turning malicious by deliberately bypassing a placed ban, you could perhaps consider this unauthorized access. Anyway, note that the user is persistent can just use any of the millions of proxy or tor hosts, get a random shared host with php shell file and bounce through it, etc. There are basically unlimited methods to bypass a ban, unless you run your site as an intranet with deny everybody, and only allow a unique set of ip addresses to join (or whatever).
I think it's bad practice to ban a complete range over an individual.
Btw, create a log file with a timeline of the events, linking to screenshots and .txt log files.
If abuse happens, you remember it later and have a more detailed record of it than the other party.
And if abuse continues, you grow a log file that you can use if it ever goes to court.
Make it clear to the user that you two clash, and appear to be unable to resolve it. That since it's your site you prefer to discontinue communications and that the user isn't welcome to the site anymore. That bypassing a set ban is considered unauthorized access.
That way your log file also has an email reference that you've done your best and to your upmost ability to clarify on a mature level that someone's not welcome on your site. Any attempt beyond that could be considered deliberate.
Anyway.
Don't accept the nonsense from a user, don't focus on it too much. Don't publicly fight it out or discuss it on your site.
Consequences are not yours, it's the abusive user who's simply not welcome. Ban the accounts, over and over again. Keep a log of it. Ban the IP and email addresses used, over and over again. etc.
Don't lie awake at night over some douche that is causing you problems. Sleep well at night knowing you're not doing anything wrong and that the abusive user might sleep a bit lighter than you.