Is this possible when you don't know the User Name? We have two corporate BES (4.2 SP3) and ran into this very issue.
An employee had set up a BIS account, then left the company. I had to call Blackberry support and get them to kill it. There was no way for me to access it.
I understand you can controll it via BES IT Policy -
Service Exclusivity Policy Group > Allow Other Message Services > 'False'