Privacy International has a report, ‘Dumb Design or Dirty Tricks?‘ on the practice of a number of popular websites – most notably eBay and Amazon – of lacking an easy or obvious way for a user to delete his or her account:

“Amazon provided the most blatant example of companies that refuse to provide account delete facilities… creating an account is relatively simple… However nowhere on the site can a customer actually delete an account. A trawl through all the ‘useful information’ statements (‘customer charter’, ‘privacy notice’ and ‘privacy policy’, ‘security guarantee’ and even ‘sign out from our site’) reveals nothing about closing your account, deleting your personal details, or terminating your relationship with Amazon. Even the site’s search function is useless for this: you can only search for products for purchase, not for information on how to manage your account. In fact, a search for ‘delete account’ even points to advertisements from ‘sponsors’ on how to open bank accounts.”

It is, of course, in no way ‘dumb design’, as the omission and obfuscation is entirely intentional: it is cunning design, frustrating a user’s attempts at exerting control by making it hard to leave. Just look at the efforts another high-profile name goes to for customer retention. It’s another feature deletion example, similar in spirit to, say, disabling the fast-forward button on PVRs.
(It’s unclear exactly what the immediate benefit is to Amazon or eBay to retain customers who want to leave and presumably are not going to be spending any more, except that a bigger customer base allows higher advertising rates, and also, as noted by PI: “The size of an online company’s customer base is a key element of its market value. Maintaining growth of that customer base is therefore a core indicator of their financial worth”; I suppose there is also the likelihood that customers may return at some point, and having an extant account removes one ‘hassle’ barrier to entry.)
PI believes that the absence of an easy account closure mechanism:

“breach[es] key elements of the Data Protection Act. No customer could reasonably be expected to invest the considerable time and effort required to investigate these sites, nor in our view should any responsible company create such obstacles.
…
As a consequence of this research, Privacy International has lodged a complaint with the UK Information Commissioner, requesting a formal investigation. This will be a test complaint, and has been directed at eBay.co.uk, which claims a user base of over ten million UK consumers.”

These are interesting examples of systems being designed to restrict users’ behaviour for commercial reasons, in an – on the face of it – extremely blatant way. There is some difference between a system which requires continuous payment, such as AOL, being designed to be difficult to cancel, and the eBay/Amazon examples, since the user is not locked in to paying a fee every month. But the effect for the locker-in is the same: more customers retained. There are plenty of parallels in designed-in lock-ins from other industries, from cigarettes and ink cartridges to deliberate software incompatability – even in Web 2.0 – and vendor lock-in generally.