You can turn off AutoPlay for one specific drive, not all drives, by patching the registry. This Wiki article on
AutoRun provides guidance.
I'm not going to try to explain the process. The Wiki does a much better job than I could. However, I will point out that the first step is determining which drive letter your reader is mounted as when you plug in the USB. That is the drive to turn off for AutoPlay. The other drives will not be affected.
This process gets rid of the prompt to install EBL, but it does not stop the AutoRun launch of the EBL program. I haven't been able to turn that off. I've been holding off deleting the program entirely, but I'm about out of patience. I use calibre exclusively, and don't buy from the Sony store, so I guess I'm about out of reasons to keep EBL.