I do not use Backup Assistant. I did not like it. When I initially activated my Droid X, I did not enter a PIN and skipped Backup Assistant usage. Therefore, my Droid X never connects with Backup Assistant.
With that said, can you do this...using your PC, go to Backup Assistant and log-in. Change your PIN. Save and log out.
Then, on your Droid X, do not enter the new PIN and the Droid X cannot access Backup Assistant.
Is there a different way? Let me know.
I do not believe there is any way to disable Backup Assistant from the Droid X. I have not seen anyway to do that. But, if the Droid X doesn't have the correct PIN, it will not connect with Backup Assistant.
I am sorry that the "Enable/Disable" option not available in BA. It is needed.
You may want to follow this thread http://community.vzw.com/t5/DROID-X-by-Motorola/Contacts-by-backupassistant/td-p/360746 and see what help that can be toward your issue.
I found this that might solve your issue.
The only other fix I could find is to do a factory reset and when it prompts you to set up BA just answer no.