Some perspective shifts that might prove helpful:
(1) In the grand scheme of things, malicious submittals with ulterior motives are the large majority of all submittals. But these things you mention aren't blips on the radar screen. (Most editors haven't seen such a technique even once.) The malice of submitters is far more often directed at the ODP itself rather than at competitors. And, think about THIS: suppose you, as a handcrafted widget maker, decided to do vile things to your competitors. WHICH COMPETITORS WOULD YOU CHOOSE TO BESMIRCH? (Conclusion: bless your competitors: if someone goes amok, you're likely to be saved from attack by being lost in the crowd.) But on the other side, editors are used to dealing with malice. And if a site is good enough (from a surfer's perspective) to have random surfers recommend it, I don't think its webmaster should need to worry about ... random surfers recommending it. We're all surfers in here, and it should show.
(2) There's hardly ever such a thing as a "right" submittal. Even those of listable sites nearly all go to the wrong category and/or contain noncompliant titles and/or descriptions.
We can handle them, just like we handle websites that were never suggested, by looking at the website itself, which we'd do anyway. The danger of someone going amok against precisely the one submitter in 10,000 who suggested an almost-perfect submitter, just doesn't bear worrying about.
(3) There's no such thing as an "official" submittal. Site owners cause nearly all of our problems, so obviously, given a choice, we'd prefer to keep the disinterested surfers' suggestion. But we don't discriminate against the troublesome class AS a class: all contributors have the same standing. (In fact, for years, even editors suggesting to a category outside their permissions had that same standing: now, editors' actions are listed separately so that they can receive quicker attention. But even they don't have a guaranteed priority.)
(4) In the normal course of events, the editor only needs to see one or two submittals, all the rest need to be thrown away somehow. Chances are extremely good that one of the better submittals will be kept. (This is another reason the garden-variety spammer wouldn't take this approach. Suppose, by a freak of fate, his submittal turned out to be for a competitor who hadn't yet submitted his own site: one of his submittals might turn out to be the trigger that got his competitor listed!)
(5) Pretty much, honest professionals and honest companies don't waste their time and money trying to besmirch their "competitors". Joe-Bob LeadPants, Master Plumber, isn't going to be landing any business by pushing Pipe-Wrench Jack's webpage off Google's top ten. Joe-Bob and Jack both get most of their calls from customer referrals and the Yellow Pages anyway, and they both know it. And in the online underworld, And Viagra Joe, Spammer Extraordinaire, won't try to slime Eckert Drugs or Walmart Pharmacies. He'll see Ginsing Pete, "the man with a million doorways to the world of scamedicines(TM)", as his REAL competition. So, the class of people who are minded to try this sort of thing, are likely to be sliming their own class -- in other words, sites that are extremely unlikely to be listable anyway.
So invest your worrying time and energy about something that's actually happened somewhere in the last year -- you know, like drunk drivers, rabid dogs, drunk politicians, earthquakes, mad cows, floods, sober politicians, incendiary meteorites, used car salesmen, Madonna albums, server outages, organic spinach, ... life isn't long enough to list all the things about which you should be more worried than this.
