And if you do discover that you have inadvertently hired a douchebag, fire them already.
As a consultant, I have privs to do all kinds of damage in all sorts of places to not only my company, but any number of companies I'm doing work for or have done work for in the past.
First off, I'm not a dick; even if I were leaving because the company had shafted me horrifically, rather than, as it sounds like here, because I hypothetically just found some other opportunity that suited me more... even then, even if I felt like being vindictive (unlikely), I'd still restrict any harm I thought I could get away with causing, to those people who actually deserved it, not indiscriminately start screwing things up in a way that would harm all kinds of other people who had nothing to do with whatever hypothetical thing I'm imagining the company had done. Anyone who you imagine might start deleting crap out of random databases or whatever, you should can the guy right now, cause what's stopping him from doing that *now* over some slight, real or imagined?
But secondly and far more importantly, if I were that sort of person, and I was disgruntled, do you think I'd tell you and give you a chance to lock me out? That'd be pretty stupid. If I were disgruntled and wanted to cause harm and then quit, I would obviously do it *in that order*. cause I mean... duh?