I'm not sure if in the Web Client you can turn that feature off. I've never wanted to stop my emails coming to my BB. You said you took the Limited Data Plan, I would suggest taking the Unlimited Data Plan for at least two months, case in point, read this thread: $360 web bill
Most of us got hooked on our BB's really fast, and some of did not take the Unlimited Data Plan from the beginning and got large wireless bills.
If I find a way to turn that off, I'll post it.