...
Judging by my data collection, "x" for #2 could be much higher too. I estimate at least 10 posts per hour per account, that makes hundreds of deleted posts per day per account.
...
Even deleting 1 post per day seems like abnormal behavior, so auto-nuking any newbie and, perhaps, Jr. Member, that deletes more than 2 posts in 24 hours should stop this scheme with near-zero impact on everyone else.
This rule probably shouldn't be applied to higher ranks, though, as there are legitimate reasons why someone might want to delete many of their posts all at once (e.g. - participating in a thread that turns nasty).