Just to clarify, an account that is nuked is also permanently banned. The difference is that, in case an account is nuked, it additionally gets all its posts deleted, effectively returning the account to Brand New.
Accounts can only be nuked in certain conditions:
<…> Regular mods can ban Newbies if they haven't been whitelisted by another mod, don't have Copper membership (IIRC), and have less than 150 posts or less than 30 posts if they have earned at least 1 merit.
I believe the above is the criteria for an account to be eligible for Nuking (even though the quote states the term "ban" and not "nuke").
Nuking is not mandatory if the conditions are met: if an account is eligible, it may not be nuked for example, let’s say, if it is related to some type of scam or is spreading malware. The idea is not to delete the evidence if it is worth keeping as a reference.
For further details, see:
Division of Powers.