How can I disable alt+click window dragging?

In 14.04 (or 14.10, 15.04, 15.10, 16.04), to set the default key / value:

dconf write /org/gnome/desktop/wm/preferences/mouse-button-modifier  '"<Alt>"'

You can give it a (limited number of) other value(s), but DON'T set it to none like you could in the past. Nor should you set it to an empty string, Doing either will give you some trouble getting control over your mouse back, as I found out...

The practical value of this information is quite limited, since not many alternative keys are accepted. I found only Shift and Super to be working, which I doubt will give you the solution you'd like to have.

Of course you can use dconf-editor as well:

org>gnome>desktop>wm>preferences

Today I've learned that pressing key

Super

together with

Alt + Click

circumvents (deactivates) the dragging behavior. Solved my issues at least, as the application (Inkscape) I wanted to use Alt+Click with, seems to simply ignore the additional key (Super) been pressed.

sf. https://askubuntu.com/a/445254/618422

#edit

By the way, it's also documented officially at:

https://inkscape.org/en/doc/tutorials/basic/tutorial-basic.en.html#Selecting_under_and_dragging_selected

https://inkscape.org/doc/tutorials/basic/tutorial-basic.html

You can also do this through Compiz Settings Manager.
Just install it with sudo apt-get install compizconfig-settings-manager.
The addon is called 'Move' and is located in section called 'Window Management' I believe. You can go straight to its settings with ccsm -p move.

I really recommend this application. It gives access to massive amount of system customization options.