How do I permanently disable a specific display in gdm?

My laptop has an unknown display always active in the background. It is called "unknown display" in the system settings on Ubuntu GNOME, and it was the same when I was using Unity. I have once installed Enlightenment before, and I also had to explicitly switch off that display in the settings. This was no problem for me since I only need to switch them off once. Now that I'm using Ubuntu GNOME, gdm seems to behave as if there is another display to the right, beyond the built-in display. I can easily switch it off by using Ctrl+P, but this solution is only temporary. This problem currently causes some rendering faults whenever I move my mouse before logging in, and it causes the background image to be misplaced to the right as the display manager tries to find the center of the two displays. My laptop has a VGA port and an HDMI port. I have once tried connecting my laptop to a TV through HDMI, and the result was that the "Unknown Display" disappeared and was replaced by "HDMI *something*". I would like to permanently disable the "Unknown Display" in gdm. How can I do that? **Update:** xrandr output: Screen 0: minimum 8 x 8, current 1366 x 768, maximum 32767 x 32767 LVDS1 connected primary 1366x768+0+0 (normal left inverted right x axis y axis) 309mm x 174mm 1366x768 60.0*+ 1360x768 59.8 60.0 1024x768 60.0 800x600 60.3 56.2 640x480 59.9 VGA1 disconnected (normal left inverted right x axis y axis) HDMI1 disconnected (normal left inverted right x axis y axis) DP1 disconnected (normal left inverted right x axis y axis) VIRTUAL1 disconnected (normal left inverted right x axis y axis) LVDS-1-2 disconnected VGA-1-2 connected 1024x768 60.0 800x600 60.3 56.2 848x480 60.0 640x480 59.9 HDMI-1-2 disconnected 1024x768 (0x45) 65.0MHz h: width 1024 start 1048 end 1184 total 1344 skew 0 clock 48.4KHz v: height 768 start 771 end 777 total 806 clock 60.0Hz 800x600 (0x46) 40.0MHz h: width 800 start 840 end 968 total 1056 skew 0 clock 37.9KHz v: height 600 start 601 end 605 total 628 clock 60.3Hz 800x600 (0x47) 36.0MHz h: width 800 start 824 end 896 total 1024 skew 0 clock 35.2KHz v: height 600 start 601 end 603 total 625 clock 56.2Hz Update: Output of lshw -C video WARNING: you should run this program as super-user. *-display description: VGA compatible controller product: GF108M [GeForce 610M] vendor: NVIDIA Corporation physical id: 0 bus info: pci@0000:01:00.0 version: a1 width: 64 bits clock: 33MHz capabilities: vga_controller bus_master cap_list rom configuration: driver=nouveau latency=0 resources: irq:50 memory:a2000000-a2ffffff memory:90000000-9fffffff memory:a0000000-a1ffffff ioport:2000(size=128) memory:a3000000-a307ffff *-display description: VGA compatible controller product: 2nd Generation Core Processor Family Integrated Graphics Controller vendor: Intel Corporation physical id: 2 bus info: pci@0000:00:02.0 version: 09 width: 64 bits clock: 33MHz capabilities: vga_controller bus_master cap_list rom configuration: driver=i915 latency=0 resources: irq:48 memory:a3400000-a37fffff memory:80000000-8fffffff ioport:3000(size=64) WARNING: output may be incomplete or inaccurate, you should run this program as super-user.