Comment 12 for bug 1113420

I found a work around in ubuntu forum : http://ubuntuforums.org/showthread.php?t=1735533

*** quote ***
To disable mnemonics you should create (if it doesn't already exist) ~/.gtkrc-2.0. This file should contain the line gtk-enable- mnemonics = 0 (you can add other GTK settings if you'd like). Then, restart for the changes to take effect (you may be able to log out then log in instead).
***

I don't know what are mnemonics, and it might depends on which version of gtk your application is using.