Make sure you are in the root of your app and do:
svn propedit svn:externals vendor/plugins
Just delete the line(s) of the plugins you no longer want.
If you get the following error then you need to set your environment variable.
svn: None of the environment variables SVN_EDITOR, VISUAL or EDITOR is set, and no ‘editor-cmd’ run-time configuration option was found
For example, you can set the SVN_EDITOR to use Vi editor from your unix prompt like
In Windows, instead of export, it is
set SVN_EDITOR=c:/windows/notepad.exe (for notepad)
set SVN_EDITOR=”C:\Program Files\Posix Tools\bin\vi” (for Vi editor)