Some days ago i got additional VDS and wanted to install php5-fpm from dotdeb, but after i added dotdeb repository, apt started to try upgrade mysql and nginx from dotdeb. Solution to stop that:

$ cat > /etc/apt/preferences.d/dotdeb << EOF
Package: *
Pin: origin "packages.dotdeb.org"
Pin-Priority: 50
EOF

Next time if i will want to install something from dotdeb, i can use -t switch for apt-get or aptitude.