In general, the xxx @ att.blackberry.net email address is a bad one to use as a main address. If you change carriers that email address will no longer be valid. Try logging into your BIS page at ATT and going to Filters in this email account. You can add a filter like this:
Where it says "CRAZYB!TCH@???.COM you enter this person's email address you want blocked. I didn't think you could do this with the blackberry.net address, but apparently you can. Worth a try!