Please note that once a keyword is deleted, there is a chance that it can be rented out by another user. Once it is purchased by another user, it can no longer be added back.
You can cancel a keyword by clicking Keywords & Numbers on the left panel. To the right of the keyword that you wish to remove, click the drop down arrow.
In the drop down menu, you will see the option listed as "Delete Keyword" to remove it. A prompt will then ask you to confirm if you would like to delete the keyword.