A common scenario we run into on Stack Overflow is that we have a troll, spammer, or obvious sock puppet of another user and moderators decide to delete this user. We go on about our business only to find out an hour or two later that the account we just deleted has been recreated, using the same credentials, and is now back to trolling, spamming, or committing voting fraud once again.
I've seen some people recreate an account like this using the same credentials four or five times (multiple times in the last week, in fact). The only thing that stops them is if we remember to suspend the account for right before we destroy it. Accounts recreated using the same credentials as an account that was suspended will be locked into the same suspension the old account was serving.
In particular, this becomes a real problem with sock puppet accounts that have a network association reputation bonus attached to them, because the recreated accounts keep getting that 100 point bonus and can immediately be used for voting fraud, to troll chat, or to spam comments. I've had to deal with quite a few puppet accounts like this by suspending them for a year before deletion.
I propose that all moderator-deleted accounts behave as if they were suspended for a year upon deletion, so that recreating these accounts using the same credentials will cause the new account to be locked in a one-year suspension. This would prevent the new accounts from being used to abuse the site in the same manner as their old account did.
This won't put an end to people creating new spam, troll, or puppet accounts after we delete them, but it will at least slow them down by forcing them to create new login credentials each time. It also would curb abuse of account association bonuses by puppets.