I have problem with this function because i work in Maya and other 3D applications that use this key combination... This option on Ubuntu 11.04 was in "window" section so i could change it to something else to avoid conflicts... where is this located in Ubuntu 11.10?
5 Answers
It’s a Compiz plugin.
Install Compiz Config Settings Manager (sudo apt-get install compizconfig-settings-manager in a terminal) and open it.
Once opened, search for a “Move Window” plugin. Select it, and disable or change the first option, “Initiate Window Move”.

For Unity 2d:
With regard to Fitoschido's answer:
Clicking on the <Alt>Button1 and just disabling it didn't work for me - it had no effect (and when restarting the computer, is was enabled again!). (I'm on Ubuntu 13.10)
What worked was clicking on the <Alt>Button1 and setting it to a different combination, for example ALT-SUPER button1.
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This worked fine for me on Ubuntu 15.10 (command in terminal):
dconf write /org/gnome/desktop/wm/preferences/mouse-button-modifier '"<SUPER>"'
This changes Mouse+Tab combination to Mouse+Super.
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I had this issue with Eagle CAD, and I learned from another forum that you can hold the Super key (windows key for me) before pressing Alt.
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In conf-editor (you can open it from terminal) go to:
/apps/metacity/general
In the "mouse_button_modifier" option change <Alt> to <Super> or something else.
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