You can disable it, actually.

Settings > General > AirDrop > turn off Use Cellular Data

That said, I don’t really see why you would disable that. It’s only a backup method for when the peer-to-peer connection fails. Unless you are sending huge files on a regular basis, I wouldn’t expect it to be worth disabling. Also, most metered plans I’ve encountered just cause your connection to be very slow after you hit the data cap.

If you are on a plan that automatically charges you excessive overage fees without warning and there is no other choice, then my condolences.