Many web sites expressly prohibit harassment and if you report it through their established mechanisms, the content and/or bully should be removed from the site in a timely manner.
Do not respond to the cyberbully except to calmly tell them to stop.
If they refuse, you may have to take additional actions.
To be sure, some web site administrators are better and quicker at this than others.
Also, please be careful not to retaliate – or do anything that might be perceived by an outsider to have contributed to the problem.