Just move the configuration file out of the folder it is in (to the Trash or anywhere else if you want to keep it).
The configuration file has an extension of ".tblk", ".ovpn", or ".conf"
The folder it is located in depends on what kind of configuration it is:
Private configurations are kept in
~/Library/Application Support/Tunnelblick/Configurations
Note the "~", which denotes your home folder. An example would be
/Users/YOUR-USERNAME/Library/Application Support/Tunnelblick/Configurations
Shared configurations are are kept in
/Library/Application Support/Tunnelblick/Shared
Deployed configurations are kept inside of the Tunnelblick.app program itself:
/Applications/Tunnelblick.app/Contents/Resources/Deploy
To get to "Contents", command-click on Tunnelblick.app (not an alias to it) and select "Show Package Contents"
You will need a computer administrator's username/password to move Shared or Deployed configurations.