From The Document Foundation Wiki
Delete a variable
- delete all the setting and input fields for the variable in the text using the Del key;
- open the variable dialog box ( ▸ ▸ ▸ menu);
- select the field type Set variable;
- select the variable you want to delete;
- click the red cross. If the red cross is not available, that means that some settings or input fields for the variable are still present in the text.