Please firstly make sure that if the tag you wish to remove has been removed from your contacts - to do this you can remove a tag in bulk from your contacts, please click here to see how.
When the tag(s) has been removed from your contact then if you no longer need the tag(s) you can remove the tag name completely from inside the Global Tags box.
Access this from the Settings icon:
Within Global Settings you will see your Global Tags listed. If the tag is no longer required (as shown in the screenshot below) just delete the tag group from your tags list.
Delete any tags you no longer require from within the Global Tags box.
Click on Update Settings when you have finished.