Well, I can see one tiny possibility... if Voom were to intentionally disable the OTA tuners on purchased boxes after shutdown, and it could be proven in court that it would have been trivially easy for them to have left them enabled, there might be two possible legal avenues available (both would have to be class-action in order to make any real financial sense)
* owners could ask a court to pierce the corporate veil and hold Cablevision liable for an intentional act of vandalism committed against the STB owners' private property and seek cash damages.
* owners could ask a court for equitable relief to force Cablevision to either buy back the receivers for the purchase price less what the owners would have paid in monthly rent, or force Voom to somehow re-enable the boxes (at that point, it would probably be a moot point, since 2-3 years from now HD OTA tuners will cost < $150 and nothing short of issuing new cards to authorize the boxes would likely be feasible since the broadcast infrastructure currently used for Voom will have long since been repurposed). The nice thing about equity is that you don't have to prove any laws or contracts were violated... you just need to show that the defendant was an a$$hole for no good reason and unfairly caused needless injury or injustice.
Really, the smart thing for Voom to do would be to just leave the OTA tuners enabled, then send everyone who leased one or more boxes a prepaid mailer with the option to just buy the box (as an OTA tuner) for $25-50 (billing anyone who failed to return or buy the box after 3 months and 3+ nastygrams some fictitious overinflated price). The $25-50 Cablevision would make would probably be more than they'd end up with if they had to pay shipping via FedEx/UPS/DHL, inventorying, storage, and sale to some surplus liquidator... and they'd leave behind people who were disappointed about losing Voom, but at least satisfied that they got a nice going-away present.
Think about it. The flood of Voom boxes that would end up on eBay would probably do more to spur people who've not yet been sufficiently motivated to buy an OTA box to go out and do it... and maybe buy one or two extras so they could watch DVD-quality OTA programming on their non-HDTVs. It would go down in history as Charles Dolan's ultimate legacy, bringing HDTV into the homes of people whom up to now haven't considered it as a big deal, and getting the word out that even NORMAL TVs look better with HDTV tuners (at $250/pop, it's hard to justify... at $50-100, it's fairly easy).
I think that Cablevision's handling of the boxes post-shutdown OTA capabilities will be a good indication of who's calling the shots. If Dolan's in charge, they'll likely leave the tuners enabled, and might even just write off the boxes altogether. If the rest of the board is firmly in control, they'll likely do the Corporate America "Send in the Stormtroopers" strategy that will end up costing them more than they'd have made by just walking away AND piss off just about everyone possible.