First off I HATE IT when people just stop responding to messages! I have had it happen several times on OKC... Its so lame and shallow.... OKC is flakey girl central.
In the case of the woman you're not interested in, as long as you haven't talked specifics, and you don't mind the occasional pen-pal email... why not just keep going at that rate? If she comes right out and asks something specific you can then just say something along the lines of you just don't feel that kind of connection.
In my search I met a woman a few hours away that "could" fit the arrangement I want... but there is like no attraction at all. But at the same time I like to talk to her now and then about randomness too. Online dating is just like real dating... sometimes there just isn't a connection on that level and you just end up meeting a new person, friend if you're really lucky.
As far as video store girl goes.... I again think its incredibly rude to just ignore someone after they took the time to write you an email.... grrr, but you could just "bigger" person and continue renting videos there like nothing ever happened and just treat her like you always have. I really doubt she is going to bring it up. Did you get into specifics with her?
Then again there is always netflix...