Again: you had better backup your files and do a clean install of Trisquel 7;
without adding any proprietary software.
And I see no reason why you would not be able to remove your signature.
Unless you are paid to do advertisement for proprietary software here, in
which case you are not
It is /etc/apt/sources.list and you can use any text editor (here GEdit):
$ gksu gedit /etc/apt/sources.list
Or you can use the "Software & Updates" utility in the "System Settings".
But you had better backup your files and do a clean install of Trisquel 7.
Without adding any proprietary
