1. Disable extension
You can completely disable an extension by removing or renaming its xml file from the app/etc/modules folder. The naming convention for this file is <COMPANY_NAME>_<EXTENSION_NAME>.xml
To rename the extension, you should change the file-extension ending to anything but xml.
So for example, to disable Magmodules Rich Snippets, you could rename the file from:

app/etc/modules/Magmodules_RichSnippets.xml app/etc/modules/ Magmodules_RichSnippets.xml
app/etc/modules/Magmodules_RichSnippets.xml app/etc/modules/ Magmodules_RichSnippets.~xml

Renaming this file instead of deleting it allows you to re-enable the extension in the future.  So just add a ~ sign before the xml. After renaming the file, you will need to refresh your Magento cache. This can be done through Admin > System > Cache Management > Flush Cache or by deleting the contents of the var/cache folder.

Please note that disabling an extension under System > Configuration > Advanced > Disable Modules Output only disables block output, while the rest of the extension’s functionality remains enabled.