How to stop xampp mysql server?
Your MYSQL instance that is running is here /Applications/MAMP/Library/bin/mysqld (the MAMP one) and here /usr/local/Cellar/mysql/5.5.20/bin/mysqld (the one installed using Brew).

Try:

/Applications/MAMP/Library/bin/mysqladmin shutdown
/usr/local/Cellar/mysql/5.5.20/bin/mysqladmin shutdown


Then start the one you want to start. You also would want to check Library/LaunchDaemons/ for any launch scripts and use something like

sudo launchctl unload -w /Library/LaunchDaemons/org.macports.mysql.plist


Update: Please list the output of following commands: 
    ls /System/Library/LaunchDaemons/ | grep mysql 
    ls /Library/LaunchDaemons/ | grep mysql 
    ls ~/Library/LaunchDaemons/ | grep mysql