How do I block AirDrop?

It's only active when you select it from Finder's sidebar. Otherwise the ad hoc network is turned off.

AirDrop is only activated when you open it in Finder, and the others can't access to your computer because they need your permission

If it is a concern you can disable AirDrop:

Applications/Utilities/Terminal.app

Open a terminal session and at the prompt, copy and paste:

    defaults write com.apple.NetworkBrowser DisableAirDrop -bool YES

To turn AirDrop back on:

Open a terminal session and at the prompt, copy and paste:

  defaults write com.apple.NetworkBrowser DisableAirDrop -bool NO

Logout and log back in to see the changes.

Thank you. As a MCX profile that blocks airdrop across all machines. Nice!

(And, yes, I am aware that you can block airdrop in Restrictions, but that has funky side-effects!)