You can delete custom tags that you don't need anymore. You cannot delete system tags.
To delete a custom tag:
On the Tags page, do one of the following:
For a single tag, click the options menu at the end of the row and select Delete Tag. The tag is deleted automatically.
For multiple tags, select the checkbox next to each tag name, click More Options at the top of the last column and select Delete Tags. The tags are deleted automatically.