Could you not generate some 'rule' that would only delete accounts that has been totally inactive for say three years?
I am no techie - have no idea what would be involved, but maybe you could generate something that deletes an account if user has neither logged in nor their ISP lurked for 36 months. Send one email to registered email that says you can re-register anytime. Three years no lurking and no loggin in would indicate death or absolute disengagement.
Although then again I suppose you'd have to re-issue all T&Cs to all members again before you could instigate this wouldn't you?