i think that cant be undone on google as zhao said, unless you request manual removal.
for the future you can type on the table in the database a value that dont exist as parent id of category. that leads for the category not appears listed on the site.
Welcome to the Q&A site for Question2Answer.
If you have a question about Q2A, please ask here, in English.
To report a bug, please create a new issue on Github or ask a question here with the bug tag.
If you just want to try Q2A, please use the demo site.
April 20, 2021: Q2A 1.8.6 released!