Users who do not have the permissions to add their own keywords use the keyword list in the dictionary. Users select the appropriate keywords from the dropdown menu.
If a keyword is deleted from the dictionary, then this simply means that the keyword can no longer be added to media in the future. All media items retain their tags, even if these have been deleted from the dictionary, and remain searchable.
If you want to remove a keyword from media items themselves, you can remove them from File info > Keywords by clicking on the X next to the keyword you wish to remove.
The same applies to sets: First search for the keyword you wish to delete and then select the media items found. Next, click on Selected > Edit, navigate to "Tags" tab and remove the keyword you want to delete.
Save by clicking the checkmark in the upper right corner of the batch editing window.