That flange device is intended to keep somebody from unplugging a J1772 charge handle from the J1772-to-Tesla adapter, as might happen if you're charging somewhere public and somebody else wants to charge, or is just a jerk who thinks it'll be amusing to inconvenience an EV owner. AFAIK, that device does nothing to secure to adapter itself. The flange device works by restricting the movement of the locking pin on the J1772 handle. Tesla's CCS1 adapter includes a similar restriction in the adapter itself, so as to prevent unplugging the CCS1 plug when the car is charging. Since the Tesla locks the charge cable (or adapter) to the car while charging, the adapter can't normally be removed, even if the J1772 plug is unplugged. (I believe that there is an exception: When temperatures are close to or below freezing, Tesla does not lock the charge port, so as to prevent problems that some owners had in the past with the locking mechanism literally freezing up. There was some discussion of this 2+ years ago, when people had problems soon after the Model 3 was released. I haven't seen much discussion of this issue since then, so it's conceivable that Tesla has found another workaround. Thus, there might be some risk of theft in winter months, but I've never heard of anybody's J1772 adapter being stolen because of this.)
I'm sure that if you gripped hard enough (which might require a tool) and yanked hard enough (again, possibly requiring a tool), you could force a J1772 (or CCS1) adapter out of a Tesla, even when it was locked to the car. That would almost certainly damage something, though -- the adapter and/or the car. I have yet to hear any stories of people who've had their J1772 adapters stolen in this way, or the more expensive CHAdeMO adapter for that matter.