I’m surprised that “delete user” comes up instead of “suspend”. If I delete them, since they aren’t suspended, they can just come back right?
Generally when I catch a spammer, I’d assume suspend/ban would make more sense 
but I’m going to guess I’m missing something I’m not understanding.
Deleting an account puts the IP, email address and URLs they may have posted into the corresponding Screened Logs.
AFAIK:
pros - the IPs and email addresses in Screened are used to help stop new accounts from registering using the same IP or email address
cons - the “paper trail” of evidence is gone
The IPs can “roll up” to block IPs within ranges and email addresses have a levenshtein setting.
Why? Then you would carry that account around forever in your site database. Why would you want a spammer in your user list, in any form?
Delete implies nothing else happens, while suspend implies blockage. That’s pretty much it. If it blocks them in the same sense from creating a dupe account, then perhaps “Delete & Blacklist” sounds more upfront of what it does. Or a tooltip.
To keep a record.
Since IP is hardly any use in identifyining misbehaving individuals, our moderators have developed Sherlock Holmes quality skills in detecting them by usage/writing patterns. Reoccurring issues, themes, slang words, typos…
They have caught a dozen offenders by comparing old and new posts. Having the user handle in the database helps one to remember the case. Some are so stupid that they use similar user handle compositioning with their new accounts.
We always suspend for 9999 days and never delete.
The delete process for spammers blocks the ip and email, so those can be viewed in Admin, Logs. There is also a staff action logged as well with more info.
That said, what you are describing is griefing, not spamming per se. different scenario.
I’ve seen a few trolls in particular who aren’t that stupid (and are far from it). I’d be posting for ages if I were to explain their methods in-depth.