There are two ways you can disable Magento extension.
Option 1: Login to Magento admin and go to system -> Configuration -> Click on Advanced under the heading ADVANCED in the left column. Select the extension that you would like to disable and click Disable from the drop-down. This will disable the modules Output.
Option 2: Using FTP option. This method is particularly useful if you are not able to login to Magento admin for any reason. Login using FTP and navigate to following folder app-> etc -> modules you will see list of extensions listed here. Select the module file for the extension that you would like to disable (in some cases you might have to edit 2 files). Download the file and edit the following line and upload it back once done editing.
This method will not allow any code execution in frontend and backend, remove any layouts and setups done.
Note: Please remember to clear your Magento cache after uploading the file..