As soon I insert USB stick, Bitdefender starts background scan. Can this be prevented?

This is a known issue with Bitdefender. The inherent problem is not that the USB storage device is scanned automatically; the real problems are that:

• when the user requests that the USB storage device be disconnected ("ejected"), Windows reports that it cannot be disconnected now (because it's in use), instead of the scan being automatically stopped;
• the Windows notification doesn't mention that it is specifically the Bitdefender scan that is preventing disconnection (which the user may not be aware of, or should have been aware of, because there's [now?] a notification displayed initially, but may have forgotten);

• it is then maybe not obvious to all users how to stop an active scan (which was automatically initiated) — it can be done by right-clicking the scan-in-progress icon in/near the system tray, then choosing Exit.

For the past ~2.5 years I had that automatic scanning set up, but this month I switched to "Ask every time" out of frustration mainly related to the first point.

However, the message that now appears is a bit weird. What on earth is "immunization" of a USB stick?

I never said yes until just now to test it. After clicking "Immunize USB" I didn't see any evidence of a scan running (for a mostly full USB stick, with ~11,000 files totalling ~14 GB). Furthermore, I could successfully disconnect ("eject") it when I requested it shortly after. But when plugged in again there was no longer any dialogue box displayed to "immunize" the drive — maybe BD had identified and remembered that USB stick as having been already given the all-clear, and not having any changes since then???