Disable menubutton in the android emulator

There is an option "hardware back/home keys". Specify the option and set it to "no" when creating the virtual device.

With updated SDK tool (R22 as of this answer), the AVD manager editor no longer expose these options on the UI. You have to edit the config.ini of your AVD manually and change the following key from

hw.mainKeys=yes

to

hw.mainKeys=no

As @Yenchi said, tick on 'hw.mainKeys'. The virtual device will look like second picture.