Disable share (email, message, airdrop) on Finder and Safari
You can't.

But you can do away with the buttons at some places at least:

Anywhere you see the "share-button" you can command-drag it out of the toolbar and it will disappear.

Also you can disable parts of it it via your system preferences:

Click on the "extensions" pane



Then disable the sharing services you don't want by unchecking their checkmark:



Lastly, there's the extension in the "today"-section of notification center.

You can easily delete it there by clicking on "Edit" and then clicking the red delete icon.