Alt+Left Click , how to remove shortcut
In your Compiz Config Settings Manager click on "Move Window" plugin. Click on the very first mouse shortcut (it should be <ALT>Button1) and Disable it.
If CompizConfig doesnt work for you, try this:
Fix using dconf Editor
Install dconf Editor:
sudo apt-get install dconf-editor
Search for dconf Editor in dash and run it, then search for mouse-button-modifier and change the key from <Alt> to <Super> or anything else. That worked for me.