Assign higher priority to PPA

Here is an example of /etc/apt/preferences

mint@mint ~ $ cat /etc/apt/preferences
Package: *
Pin: release o=linuxmint
Pin-Priority: 700

Package: *
Pin: origin packages.linuxmint.com
Pin-Priority: 700

Package: *
Pin: release o=Ubuntu
Pin-Priority: 500

You probably have to run apt-get update after creating the file.

Short answer: Set pinning-priority to less than 0 and set o=LP-PPA-*, e.g.

Package: nginx-full
Pin: release o=LP-PPA-brightbox-ruby-ng-experimental
Pin-Priority: -10

Quote from apt_preferences-manpage:

How APT Interprets Priorities
...
P < 0
prevents the version from being installed

Further information: http://manpages.ubuntu.com/manpages/raring/en/man5/apt_preferences.5.html and https://help.ubuntu.com/community/PinningHowto.