odd, vnc should not be started automatically.
I'd check which user is running vnc because if you didn't start it then why is it running.
Do
ps -A |grep Xvnc
and look at which user is running it.
Log in as that user and type
vncserver -kill :1
Make the :1 whatever was displayed when you did ps.
I'd find out which startup script (.bashrc maybe) is starting vnc and nuke it.
Edit: I just remembered, I believe port 5900 being open means that the :0 display is being exported. This is done with the vnc kernel module. If you have a line in your xorg.conf that looks like this, nuke it...
Code:
Section "Module"
...
Load "vnc"
EndSection