I use the Polish keyboard layout in OS X Leopard.
I'd like to remap right alt to right cmd and disable right alt.
KeyRemap4MacBook does have a predefined setting for changing the right command to option:

If you also want to disable the right option, you could use a private.xml like this instead:
<?xml version="1.0"?>
<root>
<item>
<name>test</name>
<identifier>test</identifier>
<autogen>__KeyToKey__ KeyCode::COMMAND_R, KeyCode::OPTION_R</autogen>
<autogen>__KeyToKey__ KeyCode::OPTION_R, KeyCode::VK_NONE</autogen>
</item>
</root>
See the source for the key code values and predefined settings.