There will be a file named uninstall.sh in /usr/local/netbeans-x.x if you installed netbeans with root privilege. If you installed it with a normal user account the netbeans-X.X folder will be in your home directory. (Here x.x refers to the version number such as 6.9 or 7.2 etc)
Open a terminal and go to netbeans' installation directory using cd command. Such as cd /usr/local/netbeans-x.x.
Use 'su' to become superuser (type in your root password).
Then execute uninstall.sh file with the command sh uninstall.sh.