How can I disable a key completely?
You can disable them in most text views by saving a property list like this as ~/Library/KeyBindings/DefaultKeyBinding.dict:

{
    "\UF700" = noop:;
    "\UF701" = noop:;
    "\UF702" = noop:;
    "\UF703" = noop:;
    "$\UF700" = noop:;
    "$\UF701" = noop:;
    "$\UF702" = noop:;
    "$\UF703" = noop:;
    "~\UF700" = noop:;
    "~\UF701" = noop:;
    "~\UF702" = noop:;
    "~\UF703" = noop:;
    "~$\UF700" = noop:;
    "~$\UF701" = noop:;
    "~$\UF702" = noop:;
    "~$\UF703" = noop:;
    "@\UF700" = noop:;
    "@\UF701" = noop:;
    "@\UF702" = noop:;
    "@\UF703" = noop:;
    "@$\UF700" = noop:;
    "@$\UF701" = noop:;
    "@$\UF702" = noop:;
    "@$\UF703" = noop:;
}


If you use bash, you can disable arrow keys (without other modifier keys) by adding this to ~/.inputrc:

"\e[A":
"\e[B":
"\e[C":
"\e[D":


If you want to disable them completely, you can add this to KeyRemap4MacBook's private.xml:

&lt;autogen&gt;__KeyToKey__ KeyCode::CURSOR_LEFT, KeyCode::VK_NONE&lt;/autogen&gt;
&lt;autogen&gt;__KeyToKey__ KeyCode::CURSOR_RIGHT, KeyCode::VK_NONE&lt;/autogen&gt;
&lt;autogen&gt;__KeyToKey__ KeyCode::CURSOR_UP, KeyCode::VK_NONE&lt;/autogen&gt;
&lt;autogen&gt;__KeyToKey__ KeyCode::CURSOR_DOWN, KeyCode::VK_NONE&lt;/autogen&gt;


This would only disable them in TextMate, TextEdit, Terminal, and iTerm, and only affect some key combinations:

&lt;only&gt;TEXTMATE, EDITOR, TERMINAL&lt;/only&gt;
&lt;autogen&gt;__KeyToKey__ KeyCode::CURSOR_LEFT, ModifierFlag::NONE, KeyCode::VK_NONE&lt;/autogen&gt;
&lt;autogen&gt;__KeyToKey__ KeyCode::CURSOR_RIGHT, ModifierFlag::NONE, KeyCode::VK_NONE&lt;/autogen&gt;
&lt;autogen&gt;__KeyToKey__ KeyCode::CURSOR_UP, ModifierFlag::NONE, KeyCode::VK_NONE&lt;/autogen&gt;
&lt;autogen&gt;__KeyToKey__ KeyCode::CURSOR_DOWN, ModifierFlag::NONE, KeyCode::VK_NONE&lt;/autogen&gt;
&lt;autogen&gt;__KeyToKey__ KeyCode::CURSOR_LEFT, VK_SHIFT | ModifierFlag::NONE, KeyCode::VK_NONE&lt;/autogen&gt;
&lt;autogen&gt;__KeyToKey__ KeyCode::CURSOR_RIGHT, VK_SHIFT | ModifierFlag::NONE, KeyCode::VK_NONE&lt;/autogen&gt;
&lt;autogen&gt;__KeyToKey__ KeyCode::CURSOR_UP, VK_SHIFT | ModifierFlag::NONE, KeyCode::VK_NONE&lt;/autogen&gt;
&lt;autogen&gt;__KeyToKey__ KeyCode::CURSOR_DOWN, VK_SHIFT | ModifierFlag::NONE, KeyCode::VK_NONE&lt;/autogen&gt;
&lt;autogen&gt;__KeyToKey__ KeyCode::CURSOR_LEFT, VK_OPTION | VK_SHIFT | ModifierFlag::NONE, KeyCode::VK_NONE&lt;/autogen&gt;
&lt;autogen&gt;__KeyToKey__ KeyCode::CURSOR_RIGHT, VK_OPTION | VK_SHIFT | ModifierFlag::NONE, KeyCode::VK_NONE&lt;/autogen&gt;
&lt;autogen&gt;__KeyToKey__ KeyCode::CURSOR_UP, VK_OPTION | VK_SHIFT | ModifierFlag::NONE, KeyCode::VK_NONE&lt;/autogen&gt;
&lt;autogen&gt;__KeyToKey__ KeyCode::CURSOR_DOWN, VK_OPTION | VK_SHIFT | ModifierFlag::NONE, KeyCode::VK_NONE&lt;/autogen&gt;
&lt;autogen&gt;__KeyToKey__ KeyCode::CURSOR_LEFT, VK_OPTION | ModifierFlag::NONE, KeyCode::VK_NONE&lt;/autogen&gt;
&lt;autogen&gt;__KeyToKey__ KeyCode::CURSOR_RIGHT, VK_OPTION | ModifierFlag::NONE, KeyCode::VK_NONE&lt;/autogen&gt;
&lt;autogen&gt;__KeyToKey__ KeyCode::CURSOR_UP, VK_OPTION | ModifierFlag::NONE, KeyCode::VK_NONE&lt;/autogen&gt;
&lt;autogen&gt;__KeyToKey__ KeyCode::CURSOR_DOWN, VK_OPTION | ModifierFlag::NONE, KeyCode::VK_NONE&lt;/autogen&gt;