Removing Trusted software providers from 'Software & Updates'

The PPA didn't work so using 'Software and Updates' I removed its entry under the 'Other Software' Tab. However, under the 'Authentication' tab, it is still listed as a Trusted software provider. I clicked remove and entered my password but it is still on the list. Please help.

In this time you can rely to terminal for list current trusted software provider, by:

sudo apt-key list

Now find your key that you want to remove, when you finish searching, run following command:

sudo apt-key del (keyid)

Where (keyid) is the actual keyid of your corresponding trusted software providers.

For example:

Liso@thinkpad:~$ apt-key list    
/etc/apt/trusted.gpg.d/nowrep-qupzilla.gpg
------------------------------------------
pub   1024R/7A2DA9DB 2011-12-05
uid                  Launchpad PPA for David Rosca

/etc/apt/trusted.gpg.d/ppsspp-stable.gpg
----------------------------------------
pub   4096R/ED0F784C 2014-08-02
uid                  Launchpad PPA for PPSSPP
Liso@thinkpad:~$ sudo apt-key del ED0F784C
[sudo] password for Liso: 
OK

Will delete PPSSPP key.